BIOLOGY 


ZOOLOGY 

GALLOWAY 


BLAKISTON'S  SCIKNCK  SKRIKS 


ZOOLOGY 


A  TEXT-BOOK   FOR 

SECONDARY   SCHOOLS,    NORMAL   SCHOOLS 
AND    COLLEGES 


BY 

THOMAS  WALTON   GALLOWAY,  PH.D. 

PROFESSOR    OF    BIOLOGY    IN   THE   JAMES    MILLIKIN   UNIVERSITY 
DECATUK,    ILLINOIS 


Second  Edition,  1Rev>ised 
240  11  [lustrations 


•PHILADELPHIA 
P.  BLAKISTON'S   SON    &    CO. 

IOI2    WALNUT    STREET 
1909 


••  -> 

COPYRIGHT,  1906,  BY  P.  BLAKISTON'S  SON  &  Co. 
COPYRIGHT,  1909,  BY  P.  BLAKISTON'S  SON  &  Co. 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER.  PA. 


PREFACE  TO  THE  SECOND    EDITION. 


The  author  desires  to  express  his  obligation  to  the  teachers 
whose  approval  has  made  necessary,  thus  early,  a  second  edi- 
tion of  the  work.  The  success  of  any  text  is,  on  last  analysis, 
due  to  the  insight  of  the  teacher  who  uses  it.  The  writer  will 
be  glad  to  welcome,  from  teachers,  suggestions  for  later 
editions. 

The  new  edition  makes  it  possible  to  remove  some  typo- 
graphical and  other  errors,  and  to  make  certain  minor  changes 
and  additions  which  it  is  hoped  will  render  the  book  more 
acceptable. 

A  slight  change  is  made  in  the  title;  and  the  publishers 
announce  a  reduction  in  the  price. 

T.  W.  GALLOWAY. 


200223 


PREFACE  TO  FIRST  EDITION 


Many  attempts  have  been  made  in  recent  years  to  determine 
what  presentation  of  Zoology  (and  Botany)  is  suitable  for  a 
first  course,  especially  in  the  secondary  schools.  The  follow- 
ing principles  may  be  said  to  represent  the  main  points  of 
agreement  among  teachers  : — 

1.  The  work  done  in  a  first  course  is  primarily  for  pupils 
who  do  not  take  a  second  course.     This  first  course  should  be 
handled,  therefore,  as  a  life-training  rather  than  as  satisfying 
an  admission  requirement. 

2.  Laboratory  work  and  field  work  are  essential,  both  to 
proper  interest  and  to  proper  results,  and  should  not  be  merely 
illustrative  of  text  or  lecture  work,  but  as  far  as  possible  should 
be  the  foundation  and  point  of  departure  of  the  lectures  and 
the  text.     No  instrumentality  open  to  the  teacher  is  better  than 
the  laboratory  as  a  means  of  securing  mental  growth  for  the 
pupils. 

3.  On  the  other  hand  it  is  equally  important  that  the  work 
shall  not  be  confined  to  the  field  and  the  laboratory.     "  There 
are  many  things  in  the  infinite  concourse  of  particulars  which 
we  cannot  afford  to  verify  by  experiment."     The  end  of  labo- 
ratory work  is  gained  for  the  elementary  student  when  he 
comes  to  appreciate  the  method  and  spirit  by  which  sound  in- 
vestigation proceeds,  has  acquired  enough  technical  skill  to  fol- 
low  elementary   investigation   on   his   own   behalf,   and   has 
learned  how  to  appreciate,  and  if  necessary  to  verify,  the  state- 
ments of  others.     It  is  as  easy  to  waste  time  in  the  laboratory 
as  in  reading  text-books. 

4.  Laboratory  directions  in  texts  should  be  suggestive  rather 
than  exhaustive.     They  should  arouse  the  interest  of  the  stu- 
dent and  direct  his  investigations  rather  than  give  him  the 
information  which  he  seeks. 

vi 


PREFACE.  Vll 

5.  Work  in  Zoology,  once  begun,  should  continue  for  a 
whole  year  if  possible. 

If  we  may  judge  from  the  diversity  of  texts  which  have  ap- 
peared in  the  last  fifteen  years,  there  are  some  important  points 
yet  undetermined.  Some  of  these  are : — 

1.  What  Zoology  is  of  most  worth?    Which  of  the  numerous 
divisions  of  the  subject  should  receive  greatest  stress  in  a  first 
course?     Should  a  preponderance  of  attention  be  given  to  the 
study  of  structure  and  to  dissections  ?     Or  to  a  comparison  of 
the  various  ways  of  performing  the  functions  necessary  to  all 
animals  ?    Or  to  the  study  of  the  classification  of  animals  into 
their  families  and  species  ?     To  the  economic  value  of  animals  ? 
Or  to  the  study  of  their  relations  to  each  other,  and  to  the  plant 
kingdom,  and  to  the  inorganic  environment?     Text-books  of 
the  last  fifteen  years   have  passed   through   several    distinct 
phases  in  the  effort  to  find  an  answer  to  these  questions. 

2.  What  proportion  of  such  a  course  should  be  given  to  the 
descriptive  and  theoretical,  and  what  to  the  practical  or  labo- 
ratory aspects  of  the  subject? 

3.  What  should  determine  the  order  of  presentation  of  the 
subject, — logic  or  expediency?  and  what  is  expedient? 

The  present  book  is  intended  as  a  suggestion  in  the  direction 
of  an  answer  to  some  of  these  questions.  The  plan  of  treat- 
ment here  recommended  has  been  followed  by  the  author  in 
his  own  classes  for  a  number  of  years.  By  its  use  he  has 
secured  good  interest  and  fine  spirit,  in  the  study  of  animals 
and  animal  life,  on  the  part  of  beginners  ranging  from  the 
third  year  of  the  preparatory  school  to  freshmen  in  the  college. 

The  following  principles  have  guided  in  the  selection  and 
the  arrangement  of  the  material  of  the  present  volume : — 

i.  A  first  course  should  really  be  a  foundation  course,  and 
as  such  should  give  the  student  a  broad  and  catholic  view  of 
the  whole  subject,  without  thereby  becoming  commonplace.  It 
should  utilize  all  the  main  departments  of  Zoology,  because 
each  department  contains  matter  which  should  be  familiar  to 
all  persons  of  ordinary  education.  Furthermore,  the  depart- 


Vlll  ZOOLOGY. 

ments  of  morphology,  physiology,  ecology,  distribution,  and 
classification  furnish  exercises  which  have  distinct.,  and  yet  com- 
plementary, pedagogical  value.  Any  single  phase  of  the  sub- 
ject, however  important  or  interesting,  gives  a  false  and  there- 
fore an  unscientific  view  of  the  wonderful  science  of  Zoology, 
unless  it  is  supplemented  by  the  others.  Therefore  the  same 
book,  if  it  is  to  serve  the  pedagogical  needs  of  beginners, 
should  contain  fairly  representative  matter  from  all  the  main 
departments  of  the  science ;  and  it  should  at  the  same  time  pro- 
vide both  for  the  descriptive  work  and  for  the  practical  work 
in  the  field  and  laboratory. 

2.  The  time  in  an  elementary  course  should  be  about  equally 
apportioned  (i)  to  laboratory  work  (chiefly  in  physiology  and 
in  the  larger  problems  of  morphology,  rather  than  in  minute 
dissection)  ;   (2)  to  field  observation  on  physiology,  life  his- 
tories, and  the  simpler  problems  of  distribution  and  life  rela- 
tions;  (3)   to  the  body  of  the  descriptive  text;  and   (4)   to 
classes  of  questions  demanding  reference  to  classical  zoological 
authorities. 

3.  The  matter  of  greater  native  interest  should  underlie  and 
sustain  that  of  less.     It  should  not,  however,  exclude  or  efface 
the  latter.     The  most  interesting  is  often  the  least  important. 

4.  The  student  must  not  be  taught  that  observation  is  the 
only  source  from  which  he  may  draw.     Too  much  of  this  im- 
pression has  arisen  from  the  necessary  appeal  for  more  and 
better  laboratory  work.     It  has  become  quite  as  necessary  for 
him  to  know  something  about  authorities   (a  word  formerly 
in  some  disrepute  among  scientists,  but  now  of  increasing  im- 
portance).    Hence  even  a  beginner's  course  in  natural  history 
should  make  large  demands  upon  the  student  in  the  matter  of 
library  work, — both  as  more  economical  of  his  time  and,  on 
the  whole,  likely  to  be  more  accurate  than  his  own  uncorro- 
borated observations.     The  interaction  of  authority  and  in- 
dividual discovery  furnishes  the  teacher  his  supreme  oppor- 
tunity in  the  development  of  the  student. 

5.  Certain  of  the  general  facts  and  principles  which  the 


PREFACE.  IX 

beginner  cannot  be  expected  to  discover  for  himself  should  be 
presented  early,  in  order  to  give  the  student  a  skeleton — or 
dimensions,  so  to  speak — in  which  he  shall  later  insert  the 
particulars  which  he  discovers.  He  must  have  this  in  order  to 
unify  his  own  results  in  the  brief  time  at  his  disposal.  The 
lack  of  this  unifying  result  is  the  ground  of  the  just  complaint 
concerning  much  of  the  unorganized  and  unrelated  laboratory 
instruction  in  the  secondary  schools. 

6.  While  it  is  necessary  to  bring  our  materials  from  various 
departments  of  Zoology  and  is  desirable  that  the  student  should 
be  able  to  recognize  whether  a  given  problem  is  primarily  one 
of  structure  or  function  or  relation,   the  total  result  of  an 
elementary  course  of  Zoology  should  be  a  sense  of  unity,  of 
continuity,   and  of  interdependence.     The  final  view  of  the 
student  should  be  of  life  and  organic  progress,  and  not  of  a 
disjointed  science,  dissected  in  the  house  of  its  friends. 

7.  The  teacher  should  have  some  latitude  in  the  choice  of 
matter  and  emphasis,  in  order  that  both  may  be  properly  suited 
to  his  equipment  and  locality.     It  should  be  impossible  for  the 
teacher  or  the  class  to  use  a  text-book  in  a  slavish,  or  parasitic 
fashion.     Therefore  a  text-book  should  contain  and  suggest 
much  more  than  one  teacher  or  one  class  can  use  in  the  time 
allowed.     This  not  only  gives  the  teacher  a  chance  (and  makes 
it  necessary  for  him)  to  mould  his  own  course,  but  causes  the 
student  to  realize  that  he  is  a  mere  beginner  when  he  has  com- 
pleted his  first  course. 

In  attempting  to  apply  these  principles  to  the  present  book 
the  author  has  made  use  of  the  following  devices: — 

i.  The  book  is  divided  into  two  portions: — (i)  a  general 
part  dealing  largely  with  broad  biological  problems  and  princi- 
ples, which  constitute  the  foundations  of  the  science  and  are 
felt  to  be  for  the  most  part,  beyond  even  the  verification  of  the 
elementary  student  (chapters  I-VIII)  ;  and  (2)  a  special  part 
(chapters  IX-XXIV),  in  which  the  various  principal  phyla 
of  animals  are  taken  up  in  succession,  beginning  with  the 
lowest.  The  purpose  has  been  to  make  this  part  particularly 


X  ZOOLOGY. 

illustrative  of  the  principles  laid  down  in  the  general  portion. 

2.  Each  chapter  of  the  general  part  contains  the  following 
elements: — (i)   the  general  statement  of  principles  or  facts; 
(2)  interspersed  with  this  are  such  practical  exercises  for  labo- 
ratory, field,  or  library,  as  have  been  found  practicable  for 
elementary  classes.     These  are  intended  to  compensate  for  the 
enforced  brevity  and  abstractness  of  definitions  and  description, 
by  causing  the  student  to  find  concrete  illustration  of  the  princi- 
ples; (3)  an  analytic  summary  of  the  most  important  general 
truths  of  the  chapter  in  outline,  at  the  close  of  the  chapter; 
and  finally  (4),  a  list  of  supplementary  topics  for  individual 
laboratory  or  library  investigation  and  report.     These  supple- 
ment and  illustrate  the  text,  and  enrich  the  review  by  introduc- 
ing a  new  view-point  and  new  matter. 

3.  In  the  chapters  of  the  special  part  each  phylum  is  intro- 
duced by  field  and  laboratory  work  on  some  representatives 
taken  as  types.     This  is  followed,  corrected  and  enlarged  by  a 
brief  discussion  of  the  typical  condition  of  the  organs  and 
functions  in  the  group  as  a  whole.     This  serves  to  unify  the 
isolated  and  local  observations  of  the  student.     Next  follows 
a  brief  statement  of  the  most  important  facts  of  .classification, 
together  with  ecological  and  economic  suggestions.     Finally, 
each  chapter  concludes  with  a  list  of  supplementary  questions 
calling  for  field,  laboratory,  and  library  work  in  review,  and  as 
a  brief  view  of  new  material. 

4.  The  figures  are  carefully  selected, — the  majority  of  them 
being  specially  made  for  this  book.     With  each  figure  of  special 
moment  is  a  brief  list  of  queries  designed  to  assist  the  student 
in  the  study  of  the  figure.     It  is  a  common  complaint  among 
teachers  that  it  is  difficult  to  get  students  to  appreciate  and  to 
use  illustrations  intelligently. 

5.  The  concluding  chapter  consists  of  practical  questions 
and  special  exercises  which  necessitate  a  review  by  the  student 
of  all  that  is  essential  in  the  book,  from  a  new  point  of  view. 

6    A  briefer  course  may  be  secured  by  the  omission  of  the 


PREFACE.  XI 

matter  in  fine  print,  which  is  intended  only  for  such  schools  as 
can  give  a  full  year  to  the  subject  of  Zoology. 

7.  The  headings  of  paragraphs  are  printed  in  black-faced 
type,  in  order  to  emphasize  the  analysis  of  subject  matter. 
Technical  terms  are  in  italics  the  first  time  they  appear.  The 
author  does  not  agree  that  all  technical  language  should  be 
omitted  from  even  an  elementary  course. 

The  author  extends  most  cordial  thanks  to  the  many  pub- 
lishers and  authors  whose  courtesy  enables  him  to  reproduce 
classic  illustrations  from  their  copyrighted  works.  Especially 
to  be  mentioned  are  Macmillan  &  Co.,  D.  Appleton  &  Co., 
Wm.  Blackwood  &  Sons,  Adam  and  Charles  Black,  Swan 
Sonnenschein  &  Co.,  Henry  Holt  &  Co.,  Houghton,  Mifflin  & 
Co.,  The  Open  Court  Publishing  Co.,  N.  G.  Elwert,  Leipzig; 
Dr.  Carl  Chun,  Leipzig;  The  Division  of  Publications,  Wash- 
ington, D.  C,  University  of  Minn.  Agricultural  Experiment 
Station,  Dr.  A.  Agassiz,  Dr.  George  Dimmock,  Dr.  Henry 
C.  McCook,  Professor  G.  H.  Parker.  Recognition  is  given 
to  the  sources  in  immediate  connection  with  the  figures. 

The  thanks  of  the  author  are  also  due  to  many  fellow  teach- 
ers for  suggestions  and  criticisms  during  the  progress  of  the 
work ;  but  especially  to  Dr.  Frank  W.  Bancroft  of  the  Univer- 
sity of  California  and  Professor  J.  H.  Gerould  of  Dartmouth 
College  who  made  extensive  criticisms  and  suggestions  while 
the  book  was  in  manuscript,  and  to  Dr.  J.  W.  Folsom  of  Illinois 
University  to  whose  skill  and  painstaking  is  due  whatever 
merit  the  original  drawings  may  possess.  Many  of  the  origi- 
nal photographs  were  also  made  by  Dr.  Folsom. 

T.  W.  GALLOWAY. 
JAMES  MILLIKIN  UNIVERSITY. 


TABLE  OF  CONTENTS 


CHAPTER  PAGE 

I.  INTRODUCTION    2 

II.  PROTOPLASM  :  ITS  MORPHOLOGY  AND  PHYSIOL- 
OGY    8 

III.  THE    ANIMAL    CELL  :    ITS  MORPHOLOGY    ANE 

PHYSIOLOGY    18 

IV.  FROM    THE    SIMPLE    CELL   TO    THE    COMPLEX 

ANIMAL    27 

V.  CELLULAR   DIFFERENTIATION  : — TISSUES 40 

VI.  GENERAL  ANIMAL  FUNCTIONS  AND  THEIR  AP- 
PROPRIATE ORGANS 59 

VII.  PROMORPHOLOGY    83 

VIII.  INDIVIDUAL  DIFFERENTIATION  AND  ADAPTATION  92 

IX.  A  GENERAL  PREVIEW  OF  THE  ANIMAL  KINGDOM.  135 

X.  THE   PROTOZOA 142 

XL  THE  PORIFERA   156 

XII.    THE  CCELENTERATA    .  .  .  • 165 

XIII.  THE  UNSEGMENTED  "  WORMS  " 183 

XIV.  THE  ECHINODERMATA 200 

XV.  THE  ANNULATA: — SEGMENTED  "WORMS"...  216 

XVI.  THE  MOLLUSOA.   234 

XVII.  THE  ARTHROPODA   261 

XVIII.  THE  CHORDATA:  PROTO-VERTEBRATA 308 

XIX.  THE  CHORDATA  :  VERTEBRATA 312 

XX.  PISCES    355 

XXL  AMPHIBIA   372 

XXII.  REPTILIA    380 

XXIII.  AVES 394 

XXIV.  MAMMALIA    „ 427 

XXV.  GENERAL  SUMMARY:  A  REVIEW  OUTLINE 451 

APPENDIX.     SUGGESTIONS  TO  TEACHERS 455 

INDEX    470 


ZOOLOGY 


CHAPTER    I. 
INTRODUCTION. 

1.  Nature  presents  to  man,  as  he  looks  upon  it,  a  great  and 
interesting  variety  of  material  objects.     Each  member  of  the 
race  gathers  in  his  lifetime,  by  means  of  experience  and  infer- 
ence, a  certain  limited  knowledge  of  these  objects  and  of  the 
changes  to  which  they  are  subject.    The  knowledge,  thus  col- 
lected and  systematized  in  the  course  of  the  history  of  the 
human  race,  constitutes  the  so-called  Natural  Sciences.    Every 
one  of  us,  whether  he  deliberately  chooses  or  not,  must  be  in 
some  degree  a  natural  scientist.     The  beauty  and  interest  of 
the  work  has  attracted  and  charmed  thousands  of  people  of 
all  conditions,  in  all  parts  of  the  world. 

We  commonly  speak  of  material  objects  as  either  living  or 
non-living — as  organic  and  inorganic.  The  study  of  living 
things  in  all  their  relations  we  call  Biology.  Physics  and  Chem- 
istry are  often  considered  as  clealing  exclusively  with  inor- 
ganic matter,  and  are  therefore  placed  in  contrast  with  Biology. 
Their  principles  apply,  however,  in  ttie  realm  of  living  things 
just  as  truly  as  in  the  non-living,  and  one  must  not  imagine 
because  of  this  antithesis  that  the  phenomena  of  life  can  be 
explained  apart  from  chemical  and  physical  laws.  The  term 
Biology  was  first  introduced  about  the  beginning  of  the  nine- 
teenth century,  and  is  intended  to  express  the  fact  that  plants 
and  animals  are  similar  in  their  most  essential  structure  and 
activities.  The  term  Natural  History  is  sometimes  used 
synonymous  with  Biology. 

2.  Zoology. — Owing  to  the   fundamental  likeness  of  all 
living  matter,  there  is  great  theoretical  difficulty  in  distinguish- 
ing between  the  plant  and  animal  kingdoms.     The  practical 


ZOOLOGY. 

difficulty  however  is  confined  to  the  very  lowest  and  simplest 
forms  of  life.  The  plants  and  animals  which  come  under  the 
common  observation  of  the  student  are  readily  distinguished. 
It  is  only  the  deeper  study  which  reveals  the  underlying  simi- 
larity of  all  living  objects.  The  branch  of  Biology  which 
treats  of  plants  is  called  Botany;  that  which  Heals  with  ani- 
mals, Zoology. 

3.  Purpose  of  Zoological  Study. — The  study  of  zoology 
is  valuable  to  the  general  student  because  animals  constitute 
one  of  the  most  interesting  and  important  features  of  our  sur- 
roundings, and  have  a  most  vital  bearing  upon  our  well-being. 
In  the  second  place,  it  adds  to  our  knowledge  of  the  structure 
and  activities  of  man  himself,  to  study  him  in  his  proper  rela- 
tion to  other  animals.  Finally,  its  study  demands  of  the  student 
the  use  of  the  scientific  method,  which  consists  of  observation 
of  as  many  facts  as  possible  at  first  hand,  of  comparing  and 
contrasting  these  facts  with  one  another  and  with  the  observa- 
tions of  others,  and  of  reaching  such  conclusions  from  them 
as  may  seem  legitimate.     In  common  with  the  other  natural 
sciences  it  is  thus  seen  to  have  high  educational  value,  apart 
from  the  practical  importance  of  the  knowledge  itself. 

To  the  investigator,  the  ultimate  object  of  zoological  study 
is  to  find  the  real  nature  of  animal  life  as  it  exists,  the  mode  of 
its  development,  and  the  causes  which  have  brought  it  to  its 
present  exquisite  variety  and  adjustment.  These  larger  and 
more  general  questions  constitute  what  may  be  called  theor- 
etical Zoology,  or  the  principles  of  Zoology. 

4.  Practical  Exercises. — Cause  the  student  to  select  ten  or  more  kinds 
of  wild  animals  with  which  he  is  partially  acquainted,  and,  from  his  obser- 
vation and  experience,  to  enumerate  the  points  at  which  they  touch  human 
welfare.    Are  they,  in  each  instance,  to  be  classed  as  helpful?  as  harm- 
ful ?  or  merely  as  indifferent  ?     Is  their  influence  upon  man's  interest  direct 
or  indirect? 

What  animals  have,  in  the  past,  most  appealed  to  your  interest  ?  Select 
that  particular  quality  in  which  you  have  been  most  interested  (structure, 
powers,  instincts,  habits)  and  show  how  the  attempt  to  study  or  explain 
any  one  takes  you  at  once  into  all  the  others. 

5.  Divisions  of  the  Science. — The   facts  and  principles 
which  have  been,  and  are  yet  to  be,  discovered  concerning  ani- 


INTRODUCTION.  3 

mals  are  so  numerous  and  various  in  their  bearings,  and  in- 
vestigators approach  the  subject  from  such  different  points  of 
view  that  it  is  necessary,  in  order  to  express  these  results,  to 
divide  zoology  into  several  branches  or  departments.  It  must 
be  held  in  mind,  however,  that  these  divisions  are  more  or  less 
artificial,  and  ^  at  the  facts  of  each  department  are  to  be  con- 
sidered in  connection  with  those  of  all  the  others,  if  they  are 
really  to  be  understood.  With  all  its  departments,  animal  life 
is  to  be  thought  of  as  a  whole.  Structures  exist  for  the  per- 
formance of  function,  and  the  activities  are  intended  to  adjust 
the  animal  to  its  whole  life-relation. 

6.  Morphology  is  the  branch  of  the  science  which  deals 
with  form  or  structure  in  its  broadest  sense,  whether  internal 
or  external,  partial  or  total.  In  its  most  general  sense  it  em- 
braces the  study  of  animals  from  the  standpoint  of  symmetry, 
— that  is,  the  form  of  the  organism  with  reference  to  certain 
planes  passing  through  the  body.  For  example,  the  human 
body  may  be  so  divided  by  a  single  plane  that  two  essentially 
similar  parts  result, — the  right  and  the  left.  Again  similar 
parts  may  succeed  each  other  in  a  linear  series,  as  in  the  seg- 
ments of  the  earth-worm ;  or  they  may  radiate  from  a  central 
point,  as  in  the  arms  of  the  star-fish.  This  is  the  most  funda- 
mental kind  of  morphology.  It  relates  the  organism  to  space. 
It  is  called  Protnorphology,  and  is  related  to  Zoology  some- 
what as  the  study  of  crystals  is  to  Mineralogy. 

Anatomy  is  that  department  of  morphology  which  treats 
of  the  structure  of  parts  of  the  individual, — as  the  organs  and 
systems  of  organs,  the  tissues,  the  cells,  and  so  forth.  This  is 
known  as  gross  anatomy  if  the  study  pertains  to  the  larger 
units, — as  organs;  it  is  called  Histology,  if  the  constituent  ele- 
ments of  these  organs  (as  tissues  and  cells)  are  to  be  con- 
sidered. 

Thus  far  we  have  thought  of  structure  as  stationary  or  per- 
manent. •  As  a  matter  of  fact  we  know  that  each  organism 
begins  life  in  a  very  modest  way,  as  a  single  "  cell,"  and  grows 
more  complex  by  fairly  well-defined  stages  until  the  adult  con- 


4  ZOOLOGY. 

dition  is  reached.  The  science  of  Embryology  is  the  record  of 
this  history  of  the  successive  stages  which  the  individual  ani- 
mal assumes  in  becoming  adult,  or  at  least  until  its  organs  are 
essentially  formed. 

7.  In  Physiology  are  considered  the  facts  and  laws  relating 
to  the  activities  or  functions  of  the  organism  and  of  its  separate 
parts.     It  includes  the  tracing  back  of  the  adult  activities  to 
their  lowest  form,  as  found  in  the  simplest  animals  or  the 
youngest  stages  of  the  higher  animals.    It  includes  the  powers 
of  the  single  cell;  the  chemical  and  physical  processes  which 
seem  to  underlie  all  the  functional  activities;  the  division  and 
more  perfect  performance  of  the  primitive  functions  as  the 
various   organs   arise   and   come   to   do   their   special   work. 
Finally  it  includes  the  relation  of  the  animal  as  a  whole  to  other 
animals  of  the  same  or  of  different  species,  to  plants,  and  to 
the  inanimate  surroundings.     The  term  Ecology  is  applied  to 
this  branch  of  physiology  which  treats  of  the  relation  of  the 
organism  to  the  complex  and  wonderful  conditions  in  which 
it  finds  itself.     Of  recent  years  much  emphasis  is  being  given 
to  this  branch  of  zoology. 

8.  Animals  may  be  studied  as  to  their  distribution  or  occur- 
rence in  the  world.     For  example,  we  find  lions  in  Africa  and 
Asia  only,  and  the  African  and  Asiatic  lions  are  of  different 
varieties;  the  giraffe  is  found  only  in  Africa;  man  is  found 
over  the  most  of  the  habitable  globe,  but  before  the  era  of  easy 
communication  between  distant  countries  the  men  of  different 
regions  were  conspicuously  different.    Again  we  can  easily  see 
that  the  animals  that  live  in  the  various  bodies  of  water  are 
very  different  from  those  living  on  the  land ;  those  in  the  frigid 
zones  are  different  from  those  in  the  temperate  and  torrid. 
All  such  topics  are  treated  under  the  head  of  distribution,  or 
geographical  distribution. 

This  is  distribution  in  space.  Similarly  the  various  systems 
of  rock-strata  are  characterized  by  more  or  less  different  fossil 
remains,  indicating  a  variation  in  the  animal  life  during  the 
successive  periods  of  the  earth's  history.  This  distribution  of 


INTRODUCTION.  5 

animals  in  time  is  the  subject-matter  of  P alee o zoology.  The 
facts  of  palaeozoology  and  the  conclusions  resting  thereon  are 
among  the  most  important  in  the  whole  realm  of  Zoology, 
inasmuch  as  they  supplement  the  facts  gained  from  the  study 
of  embryology  and  morphology  of  living  species,  thus  enabling 
the  investigator  to  trace  the  history  of  the  various  races  of 
animals  into  the  remote  past. 

9.  Practical  Exercises. — Let  the  student  submit  a  written  report  on 
the  distribution  of  the  animals  in  his  immediate  neighborhood,  based  on 
his   own   observations.     The   report   need  not  be   exhaustive   in   order   to 
convince    the    student   of   the   effect   of   the    environment,   which   includes 
everything   in    the    surroundings,    on    the    distribution    of   animals.     Some 
classification  should  be  made  of  the  varieties  of  territory  included ; — as 
river,  pond,   lowland,  woodland,  prairie,   mountain,  and  the  like.     Deter- 
mine, by  reference  to  the  authorities  available,  the  geographical  distribu- 
tion of  the  following:    the  elephant,  the  camel,  the  kangaroo,  the  horse, 
the  white  bear,  the  seal,  the  salmon,  the  crocodile,  the  reef-forming  coral, 
the  sponge  of  commerce. 

10.  Classification. — In  studying  animals  and  plants  one  is 
soon  impressed  with  the  fact  that  among  the  thousands  of 
individuals,  even  of  the  same  general  kind,  there  are  no  two 
exactly  alike;  and  yet  among  them  all,  with  their  manifest  dif- 
ferences, there  are  numerous  points  of  similarity.     These  two 
facts  make  it  possible  to  group  those  most  alike  into  more  or 
less  coherent  classes,  separating  them  at  the  same  time  from 
other  classes.     The  forming,  naming,  and  defining  of  these 
groups  and  subgroups  we  call   Taxonomy  or  Classification. 
Manifestly,  true  classification  must  depend  upon  the  facts  de- 
rived from  the  completest  possible  study  of  the  structure  and 
relations  of  organisms,  and  can  only  be  perfect  when  we  know 
all  that  is  to  be  known  about  them.     In  addition  to  displaying 
our  present  knowledge  of  the  relationship  of  animals,  classifi- 
cation serves  a  most  important  end  in  giving  us  more  rapid 
power  of  using  that  knowledge  in  getting  further  knowledge 
that  is  needed. 

11.  Historical. — Zoology  as  a  science  can  scarcely  be  said 
to  be  more  than  three  hundred  years  old,  although  Aristotle, 
more  than  three  hundred  years  before  Christ,  wrote  much  of 


O  ZOOLOGY. 

value  concerning  animals.  Later  many  facts  of  general  anat- 
omy were  discovered  in  connection  with  the  study  of  medicine, 
and  about  1600  the  invention  of  the  microscope  opened  up  the 
field  of  histology.  Toward  the  end  of  the  seventeenth  century 
an  effort  was  made  to  establish  a  scientific  classification  of 
animals.  Since  that  time  very  much  of  the  attention  of  stu- 
dents of  zoology  has  been  turned  in  this  direction.  During 
the  last  century  however  there  has  been  a  constantly  increasing 
interest  in  the  study  of  embryology,  of  histology,  and  in  the 
general  theoretical  questions,  the  answers  to  which  depend 
on  the  bringing  together  of  the  results  of  studies  in  all  depart- 
ments. Such  are  the  problems  of  race  development  or  evolu- 
tion, of  heredity,  of  man's  place  in  nature,  and  the  like.  The 
most  notable  development  of  the  subject  in  recent  years  has 
been  in  connection  with  the  study  of  the  finer  structure  of  the 
cell,  in  more  exact  methods  of  studying  physiology,  and  in 
extending  its  scope  to  take  in  the  lower  organisms  as  well  as 
the  higher  and  the  single  cell  as  well  as  the  organs.  It  is 
important  to  add  that  all  this  work  is  now  being  done  in  a 
comparative  way.  The  necessity  of  comparing  the  histology, 
the  embryology,  and  the  physiology  of  one  animal  with  that 
of  another  arises  from  the  belief  in  the  unity  of  animal  life, 
and  that  all  animals  are  really  akin.  If  animals  of  different 
kinds  are  really  related,  their  likenesses  and  differences  take  on 
a  new  meaning  to  the  student,  and  classification  comes  to  ex- 
press the  degree  of  kinship,  as  well  as  to  serve  the  convenience 
of  the  investigator. 

1 1  a.  Practical  Exercises.  By  reference  to  text-books  ar- 
range a  list  of  the  most  important  zoological  discoveries,  by 
centuries.  What  great  contributions  were  made  by  the  follow- 
ing men?  Harvey;  the  Janssens;  John  Ray;  Linneus;  La- 
marck; Cuvier;  Schleiden;  Schwann;  von  Baer;  Charles 
Darwin ;  Wallace ;  Louis  Agassiz ;  Huxley ;  Weismann.  When 
and  where  did  these  scientists  live?  Mention  five  American 
zoologists  and  indicate  their  chief  work. 


INTRODUCTION.  7 

12.  Summary. 

T.  Natural  Science  embraces : 

A.  The  sciences  of  inanimate  things — 

Astronomy, 
Geography, 

Meteorology,   Mineralogy,  Lithology,   etc. 

B.  The  sciences  of  animate  things — 

Botany, 
Zoology. 

(Physics  and   Chemistry  are   fundamental  to  both 
groups  of  sciences;  Geology  embraces  portions  of 
the  subject-matter  of  both  groups.) 
II.   Subdivisions  of  Zoology. 

A.  Morphology: 

1.  Promorphology,    which    treats    of    general 
form; 

2.  Anatomy ;  =  the  structure  of  parts: 

Gross  =  structure  of  organs  and  systems 

of  organs; 
Microscopic  =   (Histology,       Cytology)  ; 

structure  of  tissues  and  cells; 

3.  History  of  Development  (structural  stages)  : 

Individual  =  (Embryology,   Ontogeny); 
Racial  =  (Phylogeny). 

B.  Physiology: 

1.  Physiology  proper ;  =  the  functional  relation 
of  part  to  part  and  to  the  whole. 

2.  Ecology ;  =  relations  of  the  individual  to  its 
whole  surroundings. 

C.  Distribution : 

1.  In  space  =  (Geographical  Distribution)  ; 

2.  In    time  =  ( Palaeozoology,    as    revealed   by 
fossils)  ; 

D.  Classification,   or  the  grouping  of  animals   ac- 
cording to  their  likeness  or  kinship. 


CHAPTER    II. 
PROTOPLASM:    ITS    MORPHOLOGY    AND    PHYSIOLOGY. 

13.  Life. — Life  may  be  thought  of  in  two  somewhat  dis- 
tinct ways.     It  may  be  considered,  first,  merely  as  an  expres- 
sion for  all  the  various  activities  of  the  organism, — the  sum 
of  all  the  phenomena  of  its  existence ;  or,  second,  as  a  force  or 
form  of  energy  from  which  the  special  modes  of  activity,  as 
feeding,  growth,  motion,  and  thinking,  arise.     The  latter  is 
the  more  common  use  of  the  term,  and  yet  the  former  is  the 
only  use  of  it  which  can  be  completely  justified.     Much  of  the 
activity  of  living  things  may  be  explained  by  reference  to  the 
ordinary  physical  and  chemical  laws.     At  least  we  know  that 
these  latter  processes  underlie  all  the  actions  which  we  call 
vital.     It  is  indeed  a  question  whether  all  vital  phenomena  are 
not  finally  to  be  explained  by  means  of  them,  without  the  need 
of  assuming  any  special  vital  principle  or  force.    There  seems, 
however,  a  growing  disposition  among  scientists  to  admit  that 
the  action  of  chemical  force  does  not  suffice  to  explain  all  the 
phenomena  of  the  living  animal.    Whether  this  is  true  or  not, 
it  is  often  convenient  to  speak  of  "  vital  force  "  as  if  it  were  a 
cause  embracing  more  than  is  usually  included  in  the  known 
chemical  and  physical  actions. 

14.  The  Relation  of  Protoplasm  to  Life. — Whatever  life 
may  be,  in  the  last  analysis,  we  never  observe  its  manifestations 
except  in  connection  with  a  substance  called  protoplasm,  which 
is  found  both  in  plants  and  animals.    Protoplasm  does  not  con- 
tain any  chemical  elements  which  are  not  found  in  other  than 
living  materials.      Notwithstanding  this   fact,  protoplasm  is 
different  from  any  other  known  substance.     It  is  more  com- 
plex and  more  highly  organized,  as  to  its  machinery,  than  any 
other  chemical  or  physical  compound  whatsoever.    Protoplasm 
has  the  power  of  growing  by  taking  up  and  changing  other 

8 


PROTOPLASM.  9 

non-living  substances ;  but,  so  far  as  we  know,  it  is  never  pro- 
duced except  as  the  result  of  the  growth  and  division  of 
antecedent  protoplasm.  The  protoplasmic  or  living  material 
in  an  organism  is  normally  composed  of  a  number  of  unit- 
masses  called  cells  (see  Chapter  III).  These  unit-masses  of 
protoplasm  are  in  some  degree  independent  of  one  another, 
because  normally  each  tends  to  form  a  wall  about  itself;  and 
yet  it  is  highly  probable  that  the  whole  protoplasm  of  an  ani- 
mal is  physically  continuous  by  means  of  delicate  connections 
between  the  units.  The  life  of  the  cells  is  not  quite  the  same 
thing  as  the  life  of  the  organism  to  which  they  belong,  for 
in  animals  composed  of  more  than  one  cell  a  cell  may  die  with- 
out involving  the  death  of  the  animal.  The  protoplasm  of  the 
cell  may  also  retain  life  for  a  time  after  separation  from  the 
living  animal  or  after  the  animal  as  a  whole  has  ceased  to  live. 
This  may  be  seen  in  the  fact  that  the  colorless  corpuscles  of 
the  blood,  which  are  regarded  as  cells  of  the  body,  may  con- 
tinue to  move  and  show  other  evidences  of  life  after  removal 
from  the  body. 

15.  Protoplasm. — The  protoplasm  is  the  strictly  living, 
active  material  of  organisms.  The  term  is  sometimes  applied 
so  as  to  embrace  non-living  substances  which  are  found .  in 
close  connection  with  that  which  is  thought  to  be  "  alive." 
Even  limiting  the  term  protoplasm  to  the  living  matter,  we 
must  avoid  regarding  it  as  a  single  substance  of  definite  com- 
position. We  should  rather  consider  it  a  very  complex  mixture 
of  substances,  each  of  which  is  a  highly  complex  compound. 

While  protoplasm  seems  fundamentally  the  same  in  plants 
and  in  animals,  there  are  yet  important  differences;  and  it  is 
probable  that  the  living  matter  of  different  animals,  and  even 
of  different  parts  of  the  same  animal,  is  diverse  in  chemical  or 
physical  structure.  This  fact,  rather  than  any  possible  dif- 
ference in  the  "  life-force  "  itself,  seems  responsible  for  the 
diversity  of  powers  of  different  organisms  and  of  different 
organs. 


10  ZOOLOGY. 

if i  Chemical  Composition  of  Protoplasm, — It  is  impossible  to  make 
factory  chemical  analysis  of  protoplasm,  as  it  loses  its  characteristic 
ami  probably  undergoes  important  chemical  and  physical  changes 
in  the  act  of  analysis.  The  dead  material  thus  obtained  is  no  longer  the 
substance  with  which  we  started,  either  as  to  its  power  or  its  structure. 
The  experiment  shows  however  that  the  substance  is  both  chemically  and 
physically  unstable.  By  an  analysis  of  the  dead  protoplasm,  we  find  pres- 
ent several  complex  organic  compounds,  known  as  proteids,  carbohydrates 
(starches  and  sugars),  fats,  ferments,  pigments,  etc.  In  addition  to  these 
are  simpler  inorganic  compounds,  as  water  and  various  salts.  Doubtless 
some  of  these  materials  are  food-substances  on  their  way  to  form  proto- 
plasm, and  others  are  the  waste-products  of  protoplasmic  disruption,  ready 
to  be  cast  out  of  the  cell.  The  proteids  are  the  most  complex  of  all  these 
substances  and  it  is  believed  that  protoplasm  finds  its  real  basis  in  these. 

The  proteids  are  various  in  composition  and  properties,  but  agree  in 
that  their  molecules  contain  carbon,  hydrogen,  oxygen,  nitrogen,  and  sul- 
phur, in  proportion  roughly  as  follows :  C  53%,  O  22%,  N  17%,  H 
7%,  S  \%.  The  white  of  egg,  the  fibrin  of  the  blood,  and  casein  in  milk 
are  examples  of  proteid. 

Carbohydrates  consist  of  C,  H,  and  O.  The  latter  elements  are  always 
present  in  the  ratio  in  which  they  are  represented  in  water  (H2O),  e.  g. 
GHjoOs.  The  starches,  sugars,  and  cotton  fibres  are  illustrations. 

The  fats  contain  the  same  elements  as  starch,  but  the  percentage  of 
oxygen  in  terms  of  the  hydrogen  is  much  smaller  than  in  the  starches. 

The  ferments  are  complex  organic  substances  which  have  the  power  of 
producing  important  chemical  changes  in  other  substances  without  being 
themselves  consumed.  They  play  an  important,  but  not  thoroughly  un- 
derstood, role  in  the  activities  of  the  organisms,  both  within  and  outside 
the  cells  which  produce  them.  The  active  principle  of  the  digestive  juices, 
as  ptyalin  and  pepsin,  are  examples  of  ferments  which  have  been  extruded 
from  the  cells. 

Water  (H2O)  is  very  important  in  both  the  chemical  and  physical 
structure  of  protoplasm.  It  is  very  variable  in  amount,  and  the  degree 
of  activity  of  the  protoplasm  is  roughly  proportional  to  the  amount  of 
water  present  Traces  of  inorganic  salts, — compounds  of  chlorine,  potas- 
sium, sodium,  calcium,  phosphorus,  iron,  etc.,  are  also  found  in  solution 
in  the  water. 

17.  The  Physical  Structure  of  Protoplasm. — This  varies 
much  from  time  to  time.  On  account  of  differences  in  the 
amount  of  water  present,  the  consistency  of  protoplasm  may 
vary  from  the  quite  fluid  condition  found  in  actively  growing 
parts,  to  the  very  much  more  solid  condition  apparent  in  dry 
seeds  and  in  the  resting  or  encysted  stage  of  some  animals. 
In  these  latter  instances  the  protoplasm  eliminates  a  large  per 


PROTOPLASM.  II 

cent,  of  its  water,  forms  a  thick  wall,  and  thereby  becomes  en- 
abled to  resist  drouth  and  heat  and  cold  as  it  could  not  possibly 
do  otherwise.  Under  ordinary  circumstances  protoplasm  ap- 
pears as  a  semi-fluid  or  gelatinous  material. 

Concerning  the  architecture  of  protoplasm  there  is  much 
diversity  of  opinion.  It  seems  probable  that  this,  like  the 
chemical  composition,  is  subject  to  considerable  variation.  It 
is  certainly  very  complicated  and  represents  at  least  two  physi- 
cally distinct  substances  mingled  in  a  very  effectual  and  won- 
derful way.  In  some  cases  at  least  these  take  on  the  appear- 
ance of  a  foam  structure  such  as  is  obtained  in  an  emulsion 
of  oil  in  water,  or  of  air  and  water  in  a  soapy  lather.  What- 
ever its  form  may  be,  it  seems  that  there  must  be  a  close  rela- 
tion between  the  architecture  and  the  powers  which  protoplasm 
shows. 

1 8.  Physiology  of  Protoplasm. — The  mass  of  protoplasm 
which  we  have  called  a  cell,  or  unit,  performs  practically  all 
the  functions  shown  by  the  more  complex  organism.     It  has 
the  power  of  feeding,  of  growth,  of  reproduction,  of  motion  in 
response  to  stimuli.     Even  in  the  higher  animals,  made  up  of 
many  of  these  units,  the  processes  are  performed,  on  last 
analysis,  by  the  individual  protoplasmic  units  of  which  the 
body  is  composed. 

19.  Irritability. — Owing  to  its  chemical  and  physical  in- 
stability,  living  protoplasm   is   constantly  changing.      These 
changes  may  be  the  direct  result  of  internal  or  external  con- 
ditions to  whose  influence  the  protoplasm  may  respond  by  a 
manifestation  of  energy  greater  than  that   involved   in  the 
stimulus.     This  quality  is  called  irritability.     It  further  seems 
that   changes    may    originate    within    the    protoplasm    itself, 
though  this  is  much  more  difficult  to  demonstrate  and  may 
merely  represent  our  ignorance  of  the  processes  occurring  in 
the  protoplasm.    This  power  is  called  automatism.    These  are 
the  most  fundamental  qualities  belonging  to  protoplasm,  and 
serve  to  make  possible  those  which  follow :  viz.,  motion,  assimi- 


12  ZOOLOGY. 

lation,  growth,  etc.  Protoplasm  varies  in  the  degree  of  irri- 
tability. In  general  it  responds  to  stimuli  most  normally  under 
those  conditions  which  are  most  favorable  to  the  ordinary  vital 
processes. 

20.  Stimuli. — All  the  disturbing  forces  or  conditions,  ex- 
ternal or  internal,  which  tend  to  cause  response  in  living  proto- 
plasm, are  called  stimuli.     The  principal  stimuli  are, — chem- 
ically active  substances,  moisture,  contacts,  heat,  light,  elec- 
tricity,   and    gravity.     Inasmuch    as    irritability    lies    at    the 
foundation  of  the  various  protoplasmic  activities  mentioned 
below,  all  the  natural  causes  which  modify  irritability,  also 
modify,  through  it,  the  vital  processes,  such  as  motion,  growth, 
etc. 

Light  affects  protoplasm  profoundly.  The  direction  of  motion  in  pro- 
toplasm is  largely  determined  by  light.  Light  may  either  attract  or  repel 
protoplasm.  Excess  of  light  retards  growth.  Heat  strongly  modifies  the 
rate  of  all  the  vital  processes.  There  is  an  optimum  temperature  at  which 
the  protoplasm  best  performs  its  work.  An  excessive  increase  or  decrease 
of  this  temperature  produces  a  cessation  of  activity,  a  condition  of  rigor, 
and  death.  The  fatal  maximum  temperature  for  ordinary  animal  proto- 
plasm may  be  said  to  be  about  45°  or  50°  C. ;  the  minimum,  o°,  or  below. 
Chemical  agents  may  stimulate  protoplasm  in  such  a  way  as  to  attract  or 
repel  organisms.  Paramecia,  which  are  single-celled  animals,  may  be  seen 
to  gather  about  an  air-bubble,  or  at  the  margin  of  the  cover-glass.  They 
will  retreat  before  an  encroaching  solution  of  certain  salts. 

It  is  a  most  significant  fact  in  this  connection  that  protoplasm  may 
become,  so  to  speak,  accustomed  to  a  stimulus  which  has  been  long  con- 
tinued, so  that  it  ceases  to  respond  in  the  customary  way.  Protoplasm 
may  gradually  be  brought,  for  example,  to  endure  and  thrive  at  a  tem- 
perature which  would  have  produced  death  if  suddenly  applied.  It  is 
almost  impossible  to  overstate  the  importance  of  this  faculty  in  enabling 
organisms  to  survive  changing  conditions.  Stimuli,  then,  may  be  said  to 
be  powerful  in  proportion  to  their  suddenness  and  intensity. 

21.  Assimilation. — The    process   of   changing   food   sub- 
stances  into   protoplasm   is   called   assimilation.      It   can   be 
effected  only  by  protoplasm.   Such   foods  may  be  relatively 
simple  substances  or  may  be  the  complex  protoplasm  of  other 
organisms.    The  protoplasm  of  the  green  leaves  of  plants  has 
the  power  of  utilizing  the  simple  inorganic  compounds,   as 


PROTOPLASM.  13 

oxygen,  water,  and  carbon  dioxide,  in  a  larger  measure  than 
that  of  animals,  which  must  have  complex  organic  foods. 

22.  Growth  and  Reproduction. — The  result  of  assimila- 
tion is  the  addition  of  new  molecules  of  complex  organic 
matter  among  the  molecules  of  the  old.  This  produces  growth. 
It  is  to  be  defined  as  increase  in  mass.  If  this  continues  in- 
definitely in  excess  of  whatever  may  tend  to  destroy  the  pro- 
toplasm, the  increase  in  size  may  lead  to  the  division  of  the 
protoplasm.  The  parts  may  separate  and  lead  an  independent 
existence.  Such  is  reproduction.  In  its  simplest  form  it  is 
merely  growth  beyond  the  limits  of  the  individual.  The  cell 
cannot  continue  to  grow  indefinitely.  Its  size  is  limited  by  the 
necessity  of  physical  support  on  the  part  of  the  soft  protoplasm, 
and  by  the  relation  between  the  outer  surface,  through  which 
the  food  must  be  taken,  and  the  volume,  which  represents  the 
mass  to  be  fed.  The  surface  increases  as  the  square  of  the 
diameter,  whereas  the  volume  increases  as  the  cube  of  the 

FIG.  i. 


FIG.  i.  Streaming  of  Protoplasm  in  the  Amoeba.  The  forward  motion  of  the, 
granules  takes  place  more  rapidly  in  the  centre  of  the  pseudopodium  (/>).  Those  at 
the  margin  fall  behind  those  in  the  centre  as  the  pseudopodium  advances. 

Questions  on  the  figure. — Why  may  the  amoeba  readily  change  its 
form?     Do  its  internal  parts  preserve  a  constant  relation  to  each  other? 

diameter.  It  is  apparent  that  the  nourishing  surface  does 
not  increase  as  rapidly  as  the  mass  to  be  nourished,  and  in  con- 
sequence the  time  will  come  when  the  nourishment  possible  to 
be  absorbed  will  just  nourish  the  volume,  and  growth  must 
cease.  This  condition  may  constitute  an  internal  stimulus  to 


14  ZOOLOGY. 

division.  At  any  rate  division  furnishes  a  way  out  of  the 
dilemma  and  allows  a  renewal  of  growth  of  the  daughter  units. 
23.  Contractility. — A  body  of  living  protoplasm  seems 
always  to  possess  the  ability  to  change  its  form  in  greater  or 
less  degree.  This  results  in  motion  of  parts  or  of  the  whole, 
and  is  called  contractility.  Movement  or  contractility  is  closely 
related  to  irritability,  and  results  from  the  action  of  stimuli, 


FIG.  2. 


—  s 


p 


FIG.  2.  The  circulation  of  protoplasm  (p)  in  a  cell  of  a  stamen-hair  of  Tradescantia. 
In  the  channels  the  granules  move  back  and  forth  to  the  various  parts  of  the  cell. 
The  remainder  of  the  cell  is  filled  with  cell-sap  (.s)  which  in  these  cells  is  colored. 

Questions  on  the  figure. — In  what  respects  are  the  activities  of  the 
protoplasm  necessarily  limited  in  this  cell  as  compared  with  the  condition 
in  Amoeba?  Why  is  circulation  an  appropriate  term? 

extenial  and  internal,  upon  the  complex  protoplasm.  It  is 
made  possible  by  the  assimilation  of  food  substances.  These, 
in  being  broken  down,  furnish  the  energy  shown  in  motion. 
The  nature  of  the  motion  resulting  from  contraction  differs 
somewhat,  depending  upon  whether  the  protoplasm  is  en- 


PROTOPLASM.  15 

veloped  by  a  cell-wall  or  is  naked.  If  without  a  wall,  it  may 
send  out  foot-like  projections  into  which  there  passes  a  stream 
of  granules,  as  in  the  Amoeba  (see  Fig.  i)  ;  if  enclosed,  the 
protoplasmic  mass  may  rotate  within  the  cell  wall,  or  there 
may  be  narrow  channels  in  which  the  currents  move  between 
banks  of  more  stationary  material.  The  latter  motion  is  de- 
scribed as  circulation.  (Fig.  2.) 

24.  Demonstrations. — The    teacher    should,    if    possible,    demonstrate 
protoplasmic  motion  to  the  students  with  a  compound  microscope  of  good 

/-magnification.    The  Amoeba  will  serve  to  illustrate  the  naked  streaming 

)•  y  motion;  Paramedum,  rotation;  the  hairs  from  the  stamens  of  Tradescan- 

^  tia  beautifully  illustrate  circulation.     (There  is  a  cultivated  species  which 

may  be  kept  blooming  in  greenhouses  at  all  seasons  of  the  year.)     Ciliary 

motion  may  be  shown  in  several  of  the  large  Protozoa,  or  by  living  cells 

scraped  from  the  oesophagus  of  the  frog. 

25.  Dissimilation. — Motion  and  the  other  responses  which 
protoplasm  makes  to  stimuli  necessarily  represent  chemical  or 
physical  changes,  or  both,  in  the  protoplasm.    It  is  well  known 
that  complex  chemical  substances,  such  as  are  found  in  proto- 
plasm, can  be  made  to  yield  energy  when  they  are  torn  down 
into  simpler  ones  by  some  element  which  has  an  affinity  for 
some  of  the  elements  constituting  the  substance.     The  result 
of  this  action  is,  always,  simpler  and  more  stable  compounds 
than  the  original,  and  therefore  of  less  use  in  the  further 
freeing  of  energy.    This  tearing-down  process  is  the  opposite 
of  assimilation  and  is  sometimes  called  dissimilation  or  katab- 
olism.    Oxygen  is  one  of  the  most  important  agents  in  nature 
for  the  freeing  of  energy  by  breaking  down  the  complex 
chemical  substances.     It  unites  with  the  carbon  particularly, 
and  this  union  is  one  of  the  principal  sources  of  energy  which 
animals  show.     The  process  is  called  oxidation  and  is  essen- 
tially the  same  thing  that  occurs  when  wood  or  coal  is  burned. 
The  energy  belonging  to  the  wood  by  virtue  of  its  chemical 
constitution  is  partly  freed  by  the  action  of  the  oxygen  in 
uniting  with  the  carbon  and  hydrogen,  reducing  the  wood  to 
ashes,  water,  and  carbon  dioxid.     In  the  stove  the  principal 
form  of  energy  secured  is  heat;  but  in  appropriate  engines, 


I 6  ZOOLOGY. 

locomotion  and  other  forms  of  mechanical  work,  or  light,  or 
electrical  energy  may  be  secured  by  the  oxidation.  So  in 
protoplasm,  various  types  of  energy  may  result  from  the  tear- 
ing down  of  the  complex  substances.  Among  these  are  animal 
heat,  motion,  nervous  energy  and  electrical  energy. 

26.  Secretion  and   Excretion.— As  a   result  of  the  constructive   and 
destructive  work  already  mentioned  as  characteristic  of  protdplasm  cer- 
tain substances,  not  themselves  protoplasm,  may  be  produced.     If  these 
products   are   of   further   use   in   the   animal   economy,   they   are  usually 
described  as  secretions;  if  they  represent  the  final  reduction  in  the  process 
of  tearing  down,  they  are  called  excretions.     Such  materials  may  be  de- 
posited either  within  the  protoplasm  or  at  its  surface.     In  the  latter  case 
it  may  be  deposited  in  a  uniform  sheet  and  produce  a  protective  mem- 
brane   (cell  wall).    The  presence  of  such  a  covering  to  the  protoplasm 
very  materially  modifies   all  the   elementary   activities   which   have  been 
described. 

27.  Demonstrations. — The   teacher   should  make  microscopic   demon- 
strations of  secretions  and  excretions : — as   starch  grains   formed  in  the 
leaves  of  plants ;  fat  in  adipose  tissue ;  cell-walls  in  plants ;  crystals  in  plant 
cells  (see  Botanies)  ;  intercellular  substance  in  cartilage  or  bone. 

28.  Supplementary  Topics   for  Library   Work. — Find   and   examine 
some   of   the   classic   definitions   of   life.     Examine   more   completely   the 
theories  of  protoplasmic  architecture.     In  what  ways  would  the  presence 
of  the  cell-wall  bring  about  modifications  of  the  protoplasmic  activities? 
Give  an  account  of  experiments  showing  the  effect  of  some  of  the  more 
important  stimuli  on  protoplasm  (as  light,  heat,  electricity).    What  of  the 
external  conditions  are  so  important  as  to  merit  the  term  "  primary  con- 
ditions of  life"?    Why  may  protoplasm  be  described  as  chemically  MM-. 
stable?    Compare  oxidation  in  the  protoplasm  with  oxidation  in  ordinary 
combustion. 

29.  Summary. — i.  Scientists  are  not  agreed  whether  life 
is  merely  the  action  of  the  ordinary  chemical  and  physical 
forces  in  connection  with  a  peculiar  substance,  or  represents 
these,  guided  by  a  type  of  energy  of  a  higher  order. 

2.  Protoplasm,  a  chemical  compound  of  exceeding  complex- 
ity and  instability,  is  the  "  physical  basis  of  life."    Differences 
in  various  living  things  are  probably  due  to  differences  in  the 
chemical  and  physical  structure  of  the  protoplasm  of  which 
they  are  composed. 

3.  Owing  to  the  unstable  character  of  the  protoplasm  it  is 
readily  -acted  upon  and  changed  by  external  forces ;  and  the 


PROTOPLASM.  17 

various  parts  of  the  protoplasm  act  on  each  other  in  such  a 
way  as  to  produce  a  display  of  energy.  The  agents  are  called 
stimuli.  Protoplasm  responds  to  stimuli  because  of  its  irri- 
tability and  contractility.  These  latter  powers  belong  natively 
to  protoplasm  because  of  its  physical  and  chemical  composition. 

4.  Protoplasmic  matter  and   the  materials  which   are  de- 
stroyed in  the  production  of  energy  are  alike  produced  by  the 
assimilation  of  food  substances  into  new  protoplasm.     This 
is  a  most  fundamental  quality. 

5.  Growth  is  increase  of  mass,  following  the  formation  of 
new  substance  by  assimilation.  The  mere  absorption  of  water 
also  results  in  growth.     Growth  leads  naturally  to  reproduc- 
tion. 

6.  Oxygen  is  one  of  the  chief  agents  by  which  the  unstable 
compounds  in  the  protoplasm  are  made  to  release  their  energy. 
The  breaking  down  of  these  compounds  leaves  unused  mate- 
rials which  must  be  excreted.     Respiration,  which  is  a  term 
applied  to  the  using  of  oxygen  and  the  elimination  of  carbon 
dioxid,  and  excretion  are  thus  seen  to  be  protoplasmic  func- 
tions immediately  connected  with  its  activity. 


CHAPTER    III. 
THE  ANIMAL  CELL;   ITS   MORPHOLOGY  AND   PHYSIOLOGY. 

30.  Introduction. — In  studying  the  structure  of  organisms 
two  methods  are  open  to  the  student  of  to-day.     He  may 
begin  with  the  whole  adult  individual  and  by  dissection  he  may 
reach  a  knowledge  of  the  constituent  parts, — organs,  tissues, 
cells.    This,  the  analytic  method,  is  the  method  of  history  and 
has  given  us  the  mass  of  details  which  we  have  at  present. 
On  the  other  hand,  it  is  possible  to  avail  one's  self  of  the 
results  of  such  studies,  to  assume  the  unit  of  structure  which 
is  uniformly  found,  and,  by  a  synthetic  process,  follow  the 
building  up  of  an  organism'  from  its  elementary  parts.     This 
is  the  process  which  the  development  of  the  individual  illus- 
trates. It  has  the  special  advantage  of  emphasizing  the  funda- 
mental unity  of  origin  of  the  organs,  and  the  likenesses  of 
organisms,  and  gives  the  true  significance  of  differentiation 
and  development. 

31.  The  Cell. — Having  discussed  in  Chapter  II  the  sub- 
stance  in  connection  with  which  life  manifests   itself,   it  is 
necessary  to  recall  the  fact  that  the  protoplasm  of  an  organism, 
while  connected  in  various  ways,  is  separated  by  boundaries 

^  into  unit-masses,  each  mass  having  the  essential  qualities  of 
the  whole.  Each  unit  mass  of  protoplasm  is  called  a  cell.  The 
cell  is  not  to  be  considered  as  the  ultimate  unit  of  structure; 
it  is  itself  a  group  of  bodies  which  are  in  turn  composite.  It 
is  thus  to  be  looked  upon  as  an  organised  structure. 

/32.  Cell   Form. — Cells,   unhampered   in   the   direction   of 
growth,  tend  to  assume  a  spherical  form.     Agencies  both  in- 
ternal and  external,  as  nutritive  processes,  tension,  pressure, 
\  \    etc.,  may  modify  this  in  such  a  way  that  almost  any  form 
)  may    be    found:    polygonal,    flattened,    elongated,    fibrous, 

(   branched,  etc. 

\  18 


THE    ANIMAL    CELL.  19 

33.  Size. — While  ordinary  tissue  cells  are  minute,  there  is 
great  variation  in  the  size  of  cells.     Many  single-celled  in- 
dividuals are  visible  to  the  naked  eye  and  egg-cells  may  be 
several  centimetres  in  diameter;  yet  many  tissue  cells  are  less 
than  .005  millimetre  in  diameter.     Cells  may  be  very  much 
extended  in  one  or  more  directions.     The  outgrowths  of  nerve 
cells  for  example  may  attain  a  length  of  several  feet,  as  when 
the  nerve  fibers  extend  from  the  trunk  to  the  tips  of  the  toes. 

34.  Structure. — The  following  parts  are  to  be  distinguished 
in  the  typical  cell: — (i)   a  general  cell  substance,  partly  liv- 


FIG.  3. 


FIG.  4. 


FIG.  3.  Diagram  showing  the  principal  parts  of  the  cell  and  something  of  the 
protoplasmic  architecture  as  it  might  appear  while  living,  a,  alveoli  or  spheres  in  the 
foam-work  (see  §  17);  c,  centrosome;  cy,  cytoplasmic  meshwork,  containing  granules; 
nu.,  nucleus;  n,  nucleolus;  v,  vacuole;  w,  cell  wall. 

FIG.  4.  Diagram  showing  principal  parts  of  the  cell  as  it  appears  when  killed 
and  stained.  The  protoplasm  shows  more  of  a  meshwork  (cy),  the  spaces  represent- 
ing the  alveoli,  f,  formed  substance  in  alveoli.  Other  letters  as  in  Fig.  3. 

Questions  on  figures  3  and  4. — If  these  cells  are  in  reality  25  M  in 
diameter,  how  much  are  they  enlarged  in  the  drawing?  (A*  is  .001  mm.). 
Identify  the  various  structures  referred  to  in  section  34. 

ing  protoplasm,  partly  non-living  matter  both  organic  and 
inorganic;  (2)  usually  a  single  highly  differentiated  nucleus 
which  contains  living  protoplasm  and  is  clearly  demarcated 
from  the  substance  about  it;  (3)  one  or  more  specialized 
bodies  known  as  centra  somes  \  (4)  a  cell  wall  or  membrane 
(Figs.  3  and  4). 
The  cell-substance  or  cytoplasm  embraces  that  portion  of 


20  ZOOLOGY. 

the  living  protoplasm  (plasma)  outside  the  nucleus,  and  the 
more  fluid  cell-sap  (chylema)  which  includes  such  non-living 
materials  as  starch,  fats,  and  inorganic  matter  dissolved  in 
water. 

35.  The    Nucleus. — The   usually   single   nucleus    lies    im- 
bedded in  the  cytoplasm  and  is  ordinarily  separated  from  it 
by  a  thin  membrane.     Nuclei  vary  greatly  in  shape,  size,  and 
degree  of  differentiation.     While  it  is  not  always  possible  to 
find  definite  nuclei  in  all  cells,  it  seems  probable  that  all  cells 
have  nuclear  material  in  one  form  or  another  at  some  stage 
of  their  history.     The  internal   structure  of  the  nucleus   is 
equally  as  complex  as  that  of  the  cytoplasm,  having  both  liv- 
ing and  non-living  portions.     It  usually  consists  of  a  network 
of  threads   (chromatin)  readily  stained  by  certain  dyes.     In 
the   meshes   of   this   a   less   easily   stainable   material   occurs 
(achromatin) ,  a  portion  at  least  of  which  is  living.     One  or 
more  deeply  stainable  bodies,  called  nucleoli,  usually  occur, 
the  real  character  of  which  is  difficult  to  estimate. 

36.  Centrosomes  or  Centrospheres. — These  bodies  lie  in 
the  cytoplasm  but  are  closely  related  to  the  nucleus,  and  ap- 
pear to  have  an  important  place  in  certain  phases  of  cell  act- 
ivity (see  "cell  division,"  §  40). 

At  such  times  the  cytoplasmic  elements  radiate  from  the 
centrosomes  in  a  very  characteristic  way  (Fig.  7,  c).  The 
influence  extends  into  the  nucleus  and  is  accompanied  by  a 
rearrangement  of  the  chromatic  elements.  The  origin  of  the 
centrosomes  is  still  a  matter  of  disagreement.  They  are  often 
spoken  of  as  attraction  spheres  from  the  fact  that  they  ap- 
pear to  exert  an  attractive  influence  upon  certain  portions  of 
the  protoplasm. 

37.  Cell-wall. — A   cell   membrane   usually   surrounds    the 
protoplasm.     It  may  be  a  non-living  organic  secretion,  or  may 
consist  of  metamorphosed  or  altered  protoplasm  in  connection 
with  such  secretion.     The  wall  is  protective  and  supportive 
in   function,   and   varies   much   in  thickness,   resistance,   etc. 


THE    ANIMAL    CELL. 


21 


Animal  cells  as  a  rule  are  not  provided  with  such  well  de- 
veloped and  resistant  walls  as  are  plant  cells. 

38.  Cell  Functions. — Since  the  cell  is  only  a  definite  mass 
of  protoplasm,  its  functions  are  in  general  those  which  have 
already  been  described  as  protoplasmic  functions.  They  are 
merely  localized  within  the  cell.  The  cell  wall  when  pres- 
ent would  naturally  modify  and  limit  in  important  ways, 
the  more  active  protoplasmic  functions.  In  such  cases  the 
independent  motion  characteristic  of  so  many  cells  must  be 
accomplished  by  special  devices.  These  frequently  take  the 
form  of  cilia  or  jlagella,  which  are  thin  protoplasmic  projec- 
tions used  after  the  manner  of  oars.  Locomotion  of  cells 
is  not  confined  to  single-celled  organisms,  but  is  found  in 
many  cells  of  the  higher  animals  and  plants — as  colorless 


FIG.  5.  Modes  of  cell  reproduction.  A,  B,  and  C,  stages  in  the  reproduction  of  the 
Infusorian,  Colpoda,  by  the  breaking  up  of  the  protoplasm  to  form  numerous  cells. 
A,  encysted  stage;  B,  protoplasm  escaping,  spores  partly  formed;  C,  spores  com- 
pletely separated  (adapted  from  Rhumbler) ;  D,  budding  in  Chlamydomyxa,  a  lowly 
Rhizopod.  b,  bud;  civ.,  cell  wall;  m,  mother  cell;  n,  nuclear  matter;  s,  spores. 

Questions  on  the  figure. — Compare  the  process  and  the  results  of  the 
two  modes  of  cell  reproduction  shown  in  this  figure.  Can  you  describe  the 
fate  of  the  "  mother  "  cell  in  the  two  cases  ? 


22 


ZOOLOGY. 


blood  cells,  sexual  cells,  etc.,  which  have  a  distinct  motion  of 
their  own.  The  muscle  cells  of  higher  animals  possess  the 
power  of  contraction  and  motion  in  a  high  degree. 

39.  Reproduction. — The  cell  grows  as  a  result  of  the  nu- 
tritive processes  and  reaches  the  limits  of  size  determined  by 
its  special  conditions.     The  internal  and  external  conditions 
constitute  a  stimulus  to  the  breaking  up  or  division  of  the 
protoplasmic   unit.     This   may   occur    ( i )    by   the   irregular 
breaking  up  of  the  protoplasm  into  numerous  masses,  each 
of  which  has  the  essential  qualities  of  the  whole  (Fig.  5,  A  and 
B)  ;  (2)  by  budding,  in  which  a  process  or  several  processes 
appear  on  the  cell,  develop  into  bodies  like  the  original  cell,  and 
finally  become  separate  from  it  (Fig.  5,  D)  ;  (3)  by  division, 
in  which  there  is  a  division  of  the  original  protoplasm  into 
two  essentially  equal  parts.     In  this  case  neither  of  the  cells 
can  be  considered  the  parent  of  the  other. 

40.  Cell  Division. — Cell  division  may  be  effected  in  either 
of  two  ways,  (a)  by  direct  or  amitotic  division,  in  which  the 

FIG.  6. 
A 


FIG.  6.  Direct  cell  division  (Amoeba).  A,  active  specimen  with  pseudopodia;  B,  be- 
coming spherical  preliminary  to 'division;  C,  beginning  of  elongation  and  constriction; 
D,  later  stage;  E,  daughter  cells  forming  pseudopodia.  ec,  clear  ectoplasm;  en,  granu- 
lar endoplasm;  f,  food  vacuole;  n,  nucleus;  ps,  pseudopodium;  v,  pulsating  vacuole. 

Questions  on  the  figure.— Why  is  this  properly  called  direct  division? 
What  structures  are  divided?  Are  the  resulting  halves  exactly  or  merely 
roughly  equal,  apparently?  Do  you  see  any  possible  gain  to  the  organism 
in  such  a  division  as  this? 


THE    ANIMAL    CELL.  23 

nucleus  and  cell  merely  constrict  into  two  nearly  equal  parts 
(Fig.  6)  ;  and  (6)  indirect  or  mitotic  division.  The  latter  is 
the  usual  method  and  is  very  complicated.  By  means  of  it  a 
very  even  division* of  the  substances  and  structures  of  the 
nucleus,  especially,  seems  to  be  secured. 

The  more  striking  stages  in  the  process  as  it  usually  occurs  are  out- 
lined in  the  text  and  figures  which  follow.  The  nucleus  will  be  seen  to 
be  especially  active 

1.  In  the   quiescent   or   resting  stage  the  structural   elements   are   dis- 
tributed in  the  way  characteristic  of  the  particular  cell  under  examina- 
tion (Fig.  7,  A}. 

2.  When  division  is  about  to  take  place,  the  chromatin  elements  in  the 
network   of  the   nucleus    assume   the    appearance   of   a   coil  or   tangle  of 
thread  (Fig.  7,  5).    The  nuclear  membrane  often  disappears  at  this  time. 

3.  The  centrosome  divides  and  the  halves  migrate  to  opposite,  poles  of 
the  nucleus,  and  from  them  as  centres  radiations  pass  into  the  cell  body 
in  all  directions.     Across  the  nucleus,  from  one  centrosphere  to  the  other, 
thread-like  lines  extend,  producing  the  appearance  of  a  spindle  (Fig.  7,  C, 
sp).     In  the  meantime  the  coil  of  chromatin  has  been  unraveled  and  has 
broken  up  into  a  definite  number  of  pieces   (chromosomes)   which  often 
form  into  V-shaped  loops.     After  certain  evolutions,  under  the  influence 
of  the  centrospheres  apparently,  these  loops  come  to  lie  in  the  equatorial 
plane  of  the  spindle,  the  apices  of  the  loops  pointing  toward  the  centre  of 
the  nucleus.     This  is  called  the  astroid  stage  (Fig.  7,  C).    The  process  up 
to  this  point  is  known  as  the  prophasc  or  preparation  stages. 

4.  Each   of  the   chromatin   loops   next   splits   longitudinally   into   two. 
This  is  the  metaphase  or  middle  stage  (Fig.  7,  D). 

5.  Each  of  these  halves  now  begins  to  move  toward  its   appropriate 
pole  or  centrosome  (Fig.  7,  £).     As  these  half-loops  leave  the  equator  and 
collect  about  the  poles  they  give  rise  to  a  double-star  appearance  or  dias- 
troid  stage  (Fig.  7,  F).    This  is  the  anaphase. 

6.  The  loops  of  chromatin  collected  at  each  pole  are  reconstructed  into 
a  coil  which  then  passes  into  the   resting  stage  at  the  new  position,  a 
membrane  is  formed,  and  the  daughter  nucleus  is  complete.    The  nuclear 
spindle  disappears,  the  radial  appearance  about  the  centrosomes,  and  even 
the  centrosome  itself,  may  disappear  or  become  inconspicuous. 

7.  Accompanying  or  following  the  last  nuclear  changes  the  cytoplasm 
may  have  become  constricted  into  two  masses,  or  separated  by  the  formation 
of  a  wall  perpendicular  to  the  axis  of  the  spindle   (Fig.  7,  G,  H}.     The 
daughter  cells  may  separate  or  remain  united.    These  final  stages  are  known 
as  the  telophase.     Cell  division  is  at  the  beginning  of  all  the  complexities 
of  structure  found  in  the  higher  forms  of  animals.    Each  sexually  produced 
organism  commences  life  as  a  single  cell,  from  which  the  adult  is  formed 
by  cell-division,  and  the  clinging  together  of  the  daughter  cells. 


24 


ZOOLOGY. 
FIG.  7. 


FIG.  7.  Indirect  or  mitotic  division  (diagrammatic);  A,  resting  mother  nucleus;  B, 
coil  stage,  with  the  centrosomes  separating;  C,  D  (metaphase),  and  E,  stages  in  the  divi- 
sion of  the  chromosomes;  F,  diastroid  (anaphase)  stage ;' G  and  H  show  the  return 
of  the  daughter  nuclei  to  the  coil  and  to  the  resting  condition,  and  division  of  the 
cytoplasm,  and  the  formation  of  the  dividing  wall:  c,  centrospheres;  cl,  chromatin 
coil;  chr,  chromosomes;  nu.,  nucleus;  n,  nucleolus;  sp,  nuclear  spindle;  w,  cell  wall. 

Questions  on  the  figure. — What  structures  possessed  by  the  original 
cell  are  divided  in  this  process?  In  what  order?  Why  is  this  termed 
"indirect"  division?  Which  is  the  more  common,  the  direct  or  the 
indirect?  Can  you  see  any  special  gain  secured  by  this  method?  Describe 
the  behavior  of  the  nucleolus  and  the  nuclear  membrane  by  comparing  this 
with  other  figures  in  reference  books. 


THE    ANIMAL    CELL.  25 

41.  Functions  of  the  Nucleus  and  Centrosomes. — While  we  can  fol- 
low some  of  the  externals   of  the  various  cell  activities,  the  manner  of 
their  occurrence  and  their  causes  are  in  the  greatest  obscurity.     We  are 
not  able  to  say  just  what  part  is  performed  by  the  different  structures 
involved.     It   is   hazardous   to   say  that  one   structure   is   more   important 
than  another;  yet  it  seeems  to  be  proven  that  the  nucleus  is  quite  essential 
in  cells  which  possess  nuclei,  for  the  proper  performance  of  even  the  ordi- 
nary nutritive  functions.     Some  of  the  unicellular  animals  may  be  artifi- 
cially mutilated  in  such  a  way  that  the  lost  parts  may  be  regenerated  and 
the   normal   form    restored.     A    relatively   small   piece   of   the    Protozoan, 
Stcntor,  for  example,  can  reproduce  the  whole,  if  a  portion  of  the  nucleus 
be  present.     A  much  larger  piece  without  nuclear  material  is  wholly  un- 
able to  regenerate  lost  parts,  and  even  seems  unable  to  control  or  exer- 
cise the  ordinary  assimilative  functions.     The  phenomena  of  indirect  cell 
division   show  that  activity  on  the  part  of  the  centrosomes  and  nucleus 
precedes  that  of  the  cytoplasm.     Experiments  also  show  that  the  division 
of  the   cytoplasm  may  be  checked   or  interrupted   by  external   influences 
without  interfering  with  the  division  of  the  nucleus.     On  the  other  hand 
nuclei  separated  from  cytoplasm  are  incapable  of  continuing  their  func- 
tions.    We  are  at  least  safe  in  saying  that  these  three  bodies,  the  centro- 
some,  the  nucleus,  and  the  cytoplasm  act  as  intracellular  stimuli  upon  each 
other,  and  that  all  are  important  in  the  work  of  the  cell. 

42.  Exercises    for    Library    and    Laboratory.— The    teacher    should 
secure  preparations  of  properly  stained  cells  showing  the  principal  struc- 
tures; also  if  possible  some  of  the  stages  of  cell  division  (see  Appendix; 
suggestions  to  teachers). 

What  are  chromosomes?  In  what  respects  and  to  what  extent  do 
nuclei  differ?  What  is  meant  by  the  "cell-doctrine"?  Give  an  outline 
of  its  history.  Compare  the  various  series  of  figures  in  your  library  illus- 
trating the  stages  of  cell  division. 

43.  Summary. 

1.  The  cell  may  be  considered  as  the  unit  of  structure,  and 
is  to  be  defined  as  a  "nucleated  mass  of  protoplasm  with  or 
without  a  cell  membrane." 

2.  The  cell  may  also  be  considered  the  unit  of  function,  in 
the  sense  that  it  embodies  all  vital  functions  in  epitome. 

3.  The  structure  of  the  typical  cell  may  be  outlined  as  fol- 
lows: 

(a)   Cell  body 

Cytoplasm — living. 
Cytolymph — non-living,  fluid. 
Metaplasm — non-living,  solid. 


26  zooLodv. 

(b)  Nucleus: 

Nucleoplasm  —  living. 

Chromatin. 

Achromatin. 

Nucleolymph  —  non-living,  fluid. 
Metaplasm  —  non-living,  solid. 
[Protoplasm  =  Cytoplasm  +  nucleoplasm.] 

(c)  Centrosome. 

(d)  Cell  wall. 

4.  In  addition  to  the  general  functions  of  protoplasm  which 
cells  possess  we  need  to  consider  in  connection  with  cells  the 
additional  functions  : 

(a)  Locomotion. 

(b)  Reproduction. 

5.  Reproduction  of  cells  occurs  by  fragmentation,  by  bud- 
ding, and  by  division.     Division  may  be  either  direct  or  in- 
direct. 

6.  The   following  diagram,   adapted   from   Flemming  wi]J 
serve  to  represent  the  stages  in  indirect  division  : 


One  mother  nucleus.  Two  daughter  nuclei, 

(a)  Resting  stage.  Resting  stage  (g   1 

(&)  Coil  stage  |  =Prophase  Coil  Stage  (f>  ~^* 

:)  Astroid  stage.  )  Astroid  stage  (e.  =  anaph 

(d)  Division  of  chromatin  loops  "=.  metaphase  (d.  / 


7.  The  important  effect  of  this  complicated  process  is,  ap- 
parently, to  secure  an  equal  division  of  the  nuclear  elements 
for   the   daughter   cells.     The   cytoplasmic   elements    in   the 
daughter  cells  may  be  strikingly  unequal. 

8.  The  exact  functions  of  the  various  structures  in  the  cell 
are  not  known.     They  cannot  be  understood  until  the  chem- 
ical and  physical  nature  of  living  protoplasm  is  known.     The 
cytoplasm,  the  centrosomes,  and  the  nucleus  seem  to  act  as 
stimuli  to  one  another,  in  assimilation,  growth,  and  division. 


CHAPTER   IV. 
FROM  THE  SIMPLE  CELL  TO  THE  COMPLEX  ANIMAL. 

44.  The  Individual  as  a  Cell-composite. — In  the  simplest 
animals,  as  the  Protozoa,  the  individual  consists  of  a  single 
cell,  and  the  life  history  of  the  individual  animal  is  such  as 
has  already  been  seen  to  belong  to  the  cell  (Chapter  III).     In 
such  an  individual  one  cannot  speak  of  organs  in  the  ordinary 
sense,  for  organs  as  we  shall  see  are  made  up  of  cells  bound 
together  in  the  doing  of  certain  work.     Yet  it  is  important  to 
remember  that  there  are  none  of  the  necessary  duties  of  life, 
such  as  getting  food,  digesting  it,  breathing,  moving,  repro- 
ducing, and  the  like, 'which  are  not  well  done  by  these  simple 
one-celled  animals.     The  many-celled  animals  agree  with  the 
simpler  ones  in  that  they  too  start  life  as  single  cells  ap- 
parently quite  as  simple  as  the  one-celled  animals  themselves. 
When  the  cells  divide,  however,  the  daughter  cells  do  not 
separate  as  in  the  Protozoa,  but  form  a  mass  of  cells  by  cling- 
ing together.     Owing  both  to  internal  and  external   forces 
the  cells  in  the  mass  do  not  long  remain  alike,  but  soon  show 
such  differences  among  themselves  as  serve  as  the  basis  for 
the  great  variety  of  structures   found  in  the  bodies  of  the 
higher  animals.     The  change  from  the  simple  cell  to  the  com- 
plex condition   in   the   adult  animals   is   not  a   sudden   one, 
but  takes   place   very   gradually   and   the   work   which   was 
formerly  done  by  the  single  cell  is   divided  up  among  the 
groups  of  different  cells  composing  the  body.     The  division 
of  the  work  to  be  done  makes  possible  and  necessary  the  spe- 
cializing of  certain  cells  to  do  each  part  of  it,  and  the  differen- 
tiation of  structures  makes  it  possible  to  do  each  separate  task 
better  than  before.    Thus  division  of  labor  and  differentiation 
of  parts  go  hand  in  hand  as  we  pass  from  the  simple  to  the 

complex  animals. 

27 


28 


ZOOLOGY. 


45.  The  Fertilized  Ovum  the  Starting  Point. — In  speak- 
ing of  the  development  of  the  adult  animal  from  the  simpler 
condition  of  the  single  cell  it  is  necessary  to  remember  that 
this  cell,  which  has  the  power  of  giving  rise  to  a  complex  in- 
dividual and  is    called  a  fertilized  ovum,  has  a  history  that  is 
very  important.     The  fertilized  ovum  represents  the  union  of 
two  distinct  cells,  known  as  germ  or  sexual  cells,  which  are 
ordinarily  quite  different  in  appearance  and  produced  by  dif- 
ferent kinds  of  individuals,  males  and  females.     Both  classes 
of  cells  may  be  produced  by  the  same  individual.     This  union 
does  not  produce  a  double  cell,  but  the  parts  of  each  seem  to 
fuse  with  those  of  the  other  in  a  very  complete  way. 

46.  The  Ovum. — The  female  germ  cell  is  known  as  the 
ovum,  and  is  typically  a  spherical  cell  with  abundant  nourish- 


FIG.  8.  Types  of  ova.  A,  primitive  amoeboid  ovum  of  Sponge;  B,  semi-diagram- 
matic figure  of  spherical  ovum  of  Sea-urchin  in  which  the  yolk  is  uniformly  distributed; 
C,  figure  of  a  spherical  ovum  (such  as  may  be  found  in  some  Worms  or  in  the  Frog)  in 
which  the  yoke  tends  to  collect  at  one  pole,  p.p.,  and  the  nucleus  and  protoplasm  at  the 
other,  a.p.;  m,  micropyle;  nu,  germinal  vesicle  (nucleus);  n,  germinal  spot  (nucleolus) ; 
y,  yolk  spheres. 

Questions  on  the  figure.— What  are  the  points  of  agreement  in  these 
three  ova?  The  chief  points  of  contrast?  What  is  the  function  of  the 
micropyle?  Is  a  micropyle  always  present  in  ova?  Why  are  the  poles  of 
the  ovum  appropriately  called  active  and  passive? 

ment  and  inactive  as  compared  with  the  male  cell.  It  often 
has  an  especially  well-developed  cell-covering.  Its  nucleus  is 
sometimes  called  the  germinal  vesicle  and  its  nucleolus,  the 
germinal  spot  (Fig.  8).  The  ovum  must  be  distinguished 


FROM    SIMPLE    CELL    TO    COMPLEX    ANIMAL. 


from  what  is  popularly  known  as  an  egg.  The  latter  term 
is  loosely  used  to  describe  the  fertilized  ovum  more  or  less 
developed,  together  with  its  nutritive  and  protective  coats 
such  as  occur  around  the  eggs  of  birds  and  reptiles.  Ova 
differ  very  greatly  in  size.  The  largest  are  found  among 
the  birds.  The  "  yellow  "  of  these  eggs  represents  the  real 
size  of  the  ovum.  Variations  in  size  are  due  not  so  much  to 
a  difference  in  the  amount  of  protoplasm  as  to  a  varying 
amount  of  food  or  yolk  in  the  cell.  The  food  may  be  uni- 
formly distributed  throughout  the  ovum,  mingled  with  the 
protoplasm,  or  it  may  collect  at  one  pole,  forcing  the  active 
protoplasm  to  occupy  the  other  pole  (Fig.  8,  C).  The  yolk 
furnishes  food  to  the  young  individual  or  embryo  in  its  early 
development. 

47.  The  Spermatozoon  or  male  element  is  ordinarily  in 
striking  contrast  to  the  female.      It  is  typically  very  small, 


FIG.  9. 


D      C 


-H- 


FIG.  9.  Types  of  spermatozoa.  A,  from  the  round  worm  (Ascaris)  with  a  cap, 
somewhat  amoeboid;  B,  from  the  Crayfish,  with  numerous  projections;  C,  from  Frog;  D, 
from  Sea-urchin,  h,  head;  m,  middle  piece;  n,  nucleus;  t,  tail  or  flagellum. 

Questions  on  the  figure.— What  are  the  chief  points  of  similarity  and 
dissimilarity  in  these  spermatozoa?  How  do  they  agree  with,  and  how 
differ  from,  the  ova  in  Fig.  8?  How  do  they  differ  from  the  average  cell? 
What  parts  of  the  structure  of  typical  cells  are  believed  to  be  represented 
in  the  sperm  cells? 


30  ZOOLOGY. 

active,  and  with  thin  protoplasmic  projections  (Fig.  9). 
Structurally,  the  typical  spermatozoon  consists  of  a  "  head  " 
piece,  a  middle  piece,  and  a  "  tail  "  or  flagellum.  The  head 
is  composed  chiefly  of  the  chromatic  material  of  the  nucleus. 
A  delicate  covering  of  cytoplasm  envelopes  the  head  and  is 
drawn  out  into  the  projection  known  as  the  tail  (Fig.  9,  D). 
The  middle  piece  has  been  variously  interpreted.  Some  re- 
gard it  as  the  achromatic  part  of  the  nucleus,  others  as  con- 
taining the  centrosome  of  the  male  cell. 

48.  Maturation  of  the  Ovum. — After  the  egg  cell  has  been 
produced  by  the  maternal  germinative  tissue,  and  before  the 
union  of  the  male  and  female  cells,  the  nucleus  of  the  egg- 
cell  approaches  the  surface  of  the  cell  and  divides  twice  in 
close  succession  by  the  indirect  or  mitotic  method  (see  Fig. 
10).     The  cytoplasm  of  the  ovum  does  not  divide  equally. 
A  small  amount  of  cytoplasm  enclosing  a  half  of  the  nuclear 
material  forms  a  bud-like  cell  on  the  surface  of  the  ovum  dur- 
ing each  of  the  two  acts  of  nuclear  division.     These  minute 
cells,  the  polar  bodies,  are  cast  off  from  the  egg  and  perish. 
They  are  to  be  regarded  as  abortive  or  vestigial  eggs.     In  the 
egg-nucleus  there  remains  therefore  only  one-fourth  of  the 
original  nuclear  material.     The  remnant  returns  to  the  cen- 
ter of  the  cell  and  is  termed  the  female  pronucleits.     It  con- 
tains only  one-half  the  number  of  chromosomes  found  origin- 
ally in  the  ovum.     This  (elimination  of  the  nuclear  material 
including  the  reduction  of  the  chromosomes  is  known  as  the 
maturation  or  ripening  of  the  ovum)    The  abortive  cells  or 
polar  bodies  are  not  known  to  have  any  function  other  than 
this  elimination  of  material.     The  nuclei  of  the  sperm  cells 
during  their   formation  undergo  a  similar  reduction  of  the 
chromosomes  but  in  a  somewhat  different  way.     In  these  are 
no  abortive  cells.     All  the  daughter  cells  form  sperm. 

49.  Fertilization. — The  union  of  a  sperm  cell  with  the 
ovum  constitutes  the  act  of  fertilization.     Often  there  is  a 
special    aperture    (micropyle)    in    the    outer    egg-membrane 


i  KOM    SIM  i-i.i     <  i  LI.    10    COM  PL!  X    ANIMAL. 

tlm»ugh  which  the  spennato/.oon  finds  rut  ranee.  I  'siially  only 
one  sperm  cell  gains  admission  to  the  interior  of  the  ovnm, 
whether  by  way  of  the  mieropyle  or  through  the  unmodified 
membrane.  (  'hanges  noi-mally  occur  in  the  membrane  as  soon 
as  one  sperm  enters,  by  which  all  others  are  excluded.  In 

FIG.  10. 


FIG.  10.  Four  slaves  in  tin-  maturation  and  fertilization  of  the  ovum  (partly  dia- 
grammatic). A,  formation  of  the  second  polar  body  and  the  entrance  of  the  sperma- 
tozoon; B,  the  male  and  female  pronuclei,  the  former  with  aster  about  the  centrosome; 
C,  nuclei  coming  together;  sperm  centrosome  has  formed  two;  D,  pronuclei  uniting  to 
form  segmentation  nucleus;  asters  producing  spindle  preparatory  to  cleavage,  e.n.,  egg 
nucleus;  p.b.,  polar  bodies;  s,  spermatozoon;  s.c.,  sperm  centrosome  and  aster;  s.n., 
sperm  nucleus;  se,  segmentation  nucleus  produced  by  the  union. 

Questions  on  the  figure. — Tn  what  respect  is  the  formation  of  the  polar 
bodies  similar  to  ordinary  indirect  cell-division?  In  what  respects  different 
from  it?  Is  there  any  difference  shown  in  the  figures  between  the  first 
and  the  second  polar  bodies?  In  what,  apparently,  does  maturation  con- 
sist? In  what  way  does  fertilization  appear  to  compensate  for  the  loss  in 
the  formation  of  the  polar  bodies? 

eggs  which  have  been  kept  too  long  or  subjected  to  unfavor- 
able conditions,  the  response  of  the  membrane  may  not  be  so 
quickly  effected  and  multiple  fertilization  may  occur.  Such 
fertilizations  may  produce  monstrosities.  The  sperm  nucleus 
is  now  called  the  male  pronuclcus.  It  migrates  toward  the 


3  2  ZOOLOGY. 

female  pronucleus  and  fuses  with  it;  thus  is  formed  the  first 
segmentation  nucleus.  With  the  addition  of  the  chromosomes 
in  the  male  nucleus  the  fertilized  ovum  contains  the  same 
number  of  chromosomes  as  before  maturation,  which  in  each 
species  of  animals  is  a  constant  number.  It  appears  that  fer- 
tilization restores  to  the  female  cell  essentially  what  it  lost  in 
the  process  of  maturation,  and  in  addition  stimulates  it  to 
active  nuclear  and  cytoplasmic  division  as  indicated  in  the  next 
paragraph.  Follow  the  process  in  Fig.  10. 

50.  Segmentation  or  Cleavage. — Following  shortly  upon 
fertilization,    if    conditions    are    favorable,    ordinary    mitotic 
nuclear  division  begins  and  the  ovum  divides  promptly  into 
2,  4,  8,  16,  etc.,  cells  (blastomeres).     The  resulting  cells  be- 
come smaller  and  smaller  with  each  division,  since  the  whole 
egg-mass   does  not   increase   appreciably   in   size   meanwhile. 
The  first  three  cleavage  planes  are  usually  perpendicular  to 
each  other.     Their  position  is  much  modified,  however,  by 
the  presence  of  food  or  yolk  substance  in  the  egg.     The  yolk 
in  general  retards  cleavage.     If  the  yolk  is  in  small  quantity 
and  is  uniformly  distributed  through  the  egg,  the  blastomeres 
will  be  about  equal  in  size  (Fig.  n,  A),  and  will  continue  to 
divide  with  practically  equal  promptness.     If  there  is  much 
of  the  yolk  it  is  not  likely  to  be  uniformly  distributed.     Under 
the  influence  of  gravity  and  internal  forces,  the  yolk  is  likely  to 
collect  at  the  lower,  and  the  protoplasm  and  nucleus  at  the  up- 
per pole  of  the  ovum  (Fig.  1 1,  B,  C).     The  protoplasmic  pole 
is  known  as  the  active  or  formative  pole  and  the  lower,  as  the 
passive  or  nutritive  pole.     The  polar  bodies  are  normally  freed 
at  the  formative  pole.     Under  these  circumstances  the  blasto- 
meres at  the  nutritive  pole  are  larger  and  divide  less  rapidly 
than   those  in  which  the  protoplasm   is   in   excess.      If   the 
yolk  is  excessive  in  amount  that  portion  of  the  ovum  in  which 
it  collects  may  be  totally  prohibited  from  dividing. 

51.  Forms   of   Segmentation. — The   conditions   suggested   above   give 
rise  to  the  following  classes  of  segmentation. 


FROM    SIMPLE    CELL    TO    COMPLEX    ANIMAL. 
FlG.   II. 


33 


FIG.  ii.  Cleavage  and  gastrulation  (not  drawn  to  scale).  The  vertical  rows  A,  B, 
C,  and  D  represent  different  classes  of  ova.  A,  an  ovum  with  little  yolk;  B,  one  with 
considerable  yolk  collected  at  the  lower  pole  (p.p.);  C,  one  with  a  large  amount  of  dense 
yolk  crowding  the  protoplasm  to  one  side  (a.p.~)  ;  D,  ovum  with  dense  yolk  collected 
at  centre.  The  numerals  (-1-4)  indicate  stages  in  cleavage  and  gastrulation:  i,  ova; 
2,  4-8  Celled  stages  of  segmentation;  3,  blastospheres,  blastula  stage;  4,  gastrula  stage. 
a,  archenteron;  a.p.,  active  pole;  bl,  blastoderm;  bp.,  blastopore;  ec,  ectoderm;  en,  ento- 
derm;  ma.,  macrospheres;  mi,  microspheres;  p.p.,  passive  pole;  s.c.,  segmentation 
cavity;  y,  yolk;  y.c.,  yolk  cells. 

Questions  on  the  figure. — What  constitutes  the  difference  between  the 
active  and  the  passive  pole?  Judging  from  the  drawings  and  from  your 
references  to  texts  does  gravity  have  any  influence  in  determining  the 
position  of  these?  Your  evidences?  Which  pole  gives  rise  to  ectoderm? 
Why  does  the  food  substance  interfere  with  segmentation?  What  is  the 
difference  between  the  segmentation  cavity  and  the  archenteron?  How 
does  the  presence  of  food  substance  modify  the  formation  of  an  archen- 
teron ? 


34  ZOOLOGY. 

A.  Total   segmentation. 

I.  Equal:  in  which  there  is  little  yolk  material,  and  that  is  well 
distributed.     (Illustrated    in    most    of    the    lower    invertebrates 
and  mammals.)     Fig.  n,  A. 

II.  Unequal:  in  which  there  is  a  moderate  amount  of  yolk  which 
accumulates   at  the  passive  pole.     The  cells  at  the   active  pole 
are  more  numerous   and   smaller   than   at  the   passive.     (Illus- 
trated in  many  mollusks  and  in  the  amphibia.)     Fig.  II,  B. 

B.  Partial  segmentation. 

I.  Discoidal :    in  which  there  is  an  excessive  amount  of  yolk,  with 
the  nucleus  and  a  small  mass  of  protoplasm  occupying  a  disc 
at  the  active  pole.     This  disc  alone  segments,  and  the  embryo 
lies  upon  the  yolk.     (Illustrated  in  the  eggs  of  fishes,  birds  and 
reptiles.)     Fig.  n,  C. 

II.  Peripheral :    in  which  an  excess  of  yolk  collects  at  the  centre 
of  the  ovum,  with  the  protoplasm  at  the  periphery.     The  divid- 
ing nuclei  assume  a  superficial  position  and  surround  the  unseg- 
mented    yolk.     (Illustrated   in   the    eggs    of   insects    and    other 
arthropods.)     Fig.   u,  D. 

52.  Blastula    and    Morula. — As    cleavage    continues    the 
blastomeres  remain  associated  in  a  spherical  mass.     The  in- 
dividual cells  project  beyond  the  general  surface  not  unlike 
the  lobes  of  a  mulberry,  and  for  this  reason  this  stage  is  called 
the  morula  or  mulberry  stage  (Fig.  n,  2).     By  the  growth 
of  the  cells  and  by  the  imbibition  of  water  the  morula  may 
become  a  hollow  sphere  of  cells  (blastula)  the  central  cavity 
of  which  is  rilled  with  fluid.     The  cavity  is  termed  the  segmen- 
tation cavity  (Fig.  ir,  s.c). 

53.  Gastrula. — In  those  eggs  in  which  the  segmentation  is 
total,  a  next  important  step  is  the  pushing  in  of  that  side  of 
the  blastula  which  corresponds  to  the  original  nutritive  pole. 
The  process  is  known  as  imagination,  and  the  product  as  a 
gastrula  (Fig.  u,  4).    It  takes  place  much  as  one  might  sup- 
pose one  side  of  a  hollow  rubber  ball  to  be  dimpled  or  infolded 
by  the  exhaustion  of  the  air  within.     The  gastrula  is  to  be  de- 
scribed as  made  up  essentially  of  two  layers  of  cells,  one  ex- 
ternal and  called  ectoderm  or  epiblast,  an4  one  within  called 
entoderm  or  hypoblast  (Fig.  11,4).     The  segmentation  cavity 
may  be  wholly  obliterated ;  in  that  case  the  entoderm  and  ecto- 
derm come  to  lie  in  contact.     The  cavity  of  the  invagination 


FROM    SIMPLE    CELL    TO    COMPLEX    ANIMAL. 


35 


of  the  gastrula  is  the  archenteron  or  embryonic  digestive 
tract;  the  opening  into  it,  that  is,  the  mouth  of  the  gastrula, 
is  the  blast  op  ore  (Fig.  n,  bp).  In  morulas  in  which  the  seg- 
mentation cavity  is  small  and  the  cells  at  the  nutritive  pole  are 
large  (Fig.  n,  C,  4)  this  simple  condition  is  much  obscured, 
and  imagination  as  described  above  becomes  impossible.  Nev- 
ertheless early  in  development  the  cells  which  produce  the  two 
primitive  layers  are  to  be  distinguished,  and  their  relations  are 
always  substantially  as  detailed.  If  the  term  gastrula  is  ap- 
plied to  these  we  have  to  say  that  they  are  formed  in  some 
other  way  than  by  ordinary  invagination. 

54.  Library  Reference. — Let  students  report  briefly  on  gastrulation  by 
overgrowth  -(epibole),  and  by  delamination.     Compare  the  results  attained 
by  the  various  methods.     Note  what  is  constant  in  the  methods  and  in  the 
results. 

55.  Germinal  Layers. — The  ectoderm  and  entoderm  have 
thus  far  been  mentioned  as  the  primary  germinal  layers  of 
cells.     Some  of  the  Invertebrates  have  only  these  two  layers, 
but  in  most  cases  a  third  mass  of  cells  comes  to  be  situated 
between  the  ectoderm  and  entoderm,  from  which  important 
organs  are  derived.     The  third  or  middle  layer   (mesoderm 
or  mesoblast)  differs  somewhat  in  its  origin  ifr  the  different 

FIG.  12. 


FIG.  12.  Modes  of  forming  mesoderm  (diagrams  modified  from  Whitman  and 
Selenka).  A  and  B,  special  mesoblasts  distinguishable  early  in  segmentation  (Annelid): 
A,  surface  view  from  active  pole;  B,  sectional  view  of  same,  ec,  micromeres  destined 
to  form  ectoderm;  en,  macromeres  destined  to  form  entoderm;  m,  primitive  mesoblast 
which  produces  the  mesoderm.  C,  amoeboid  mesodermal  cells  (c)  budding  from  ento- 
derm into  the  segmentation  cavity  (s.c.),  in  an  Echinoderm.  a,  archenteron. 


If. 

FIG.  13.  Mesoderm  formed  by  pouches  from  entoderm  after  gastrulation.  a,  primi- 
tive gut;  bp.,  blastopore;  car,  body  cavity,  formed  from  pockets  of  the  archenteron;  ec., 
ectoderm;  en.,  entoderm;  m.,  mesoderm;  m.so,  body-wall  mesoderm;  m.sp.,  visceral 
mesoderm;  s.c.,  segmentation  cavity. 

Questions  on  figures  12  and  13. — Enumerate  the  three  modes  of  meso- 
derm formation  figured  here.  In  which  type  may  the  mesoderm  be  identi- 
fied most  early  in  the  embryonic  development?  By  comparing  with  other 
texts  determine  in  what  groups  of  animals  the  mesoderm  is  formed  as  in 
Fig.  13.  What  are  the  differences  between  A  and  B  of  Fig.  13  ? 

groups  of  animals.  It  may  originate  (i)  from  the  multipli- 
cation of  a  few  special  cells  which,  before  imagination,  be- 
come distinct  from  those  that  are  to  form  ectoderm  and  ento- 
derm (Fig.  12,  A  and  B,m)\  (2)  by  means  of  isolated,  wan- 
dering cells  budded  from  the  other  two  layers,  particularly  the 
entoderm  (Fig.  12,  C ,  c)  ;  or  (3)  from  entoderm,  in  the  form 
of  pouches  or  of  solid  buds  of  cells  which  arise  from  the  walls 
of  the  archenteron  and  extend  into  the  segmentation  cavity 
(Fig.  13,  m).  In  some  instances  there  may  occur  a  combina- 
tion of  these  methods. 

56.  Ccelom. — When  the  mesoderm  develops  by  the  last 
mentioned  method,  i.  e.  by  the  evagination  of  the  wall  of  the 
primitive  gut  (Fig.  13,  m),  we  see  a  pair  of  folds,  or  a  series  of 
pockets,  the  cavities  of  which  are  at  first  continuous  with  the 
archenteron,  but  later  become  separate  from  it  and  entirely 
surrounded  by  the  mesodermic  layers.  The  outer  wall  of  the 
mesodermic  pouches  joins  the  ectoderm  and  forms  a  body 
wall,  and  the  inner  applies  itself  to  the  entodermal  wall  of  the 
gut.  The  space  between  is  the  coclom  or  body  cavity.  When 


FROM    SIMPLE    CELL    TO    COMPLEX    ANIMAL.  37 

the  mesoderm  arises  as  a  solid  mass,  instead  of  a  pocket,  the 
body  cavity  is  formed  by  the  splitting  of  the  mass  into  an 
inner  and  an  outer  portion.  When  the  coelom  is  formed  by 
several  pockets  the  cavities  of  these  may  ultimately  coalesce, 
forming  a  single  body  cavity.  Such  a  cavity  is  found  in  all 
the  vertebrates  and  in  the  higher  invertebrates,  although  it 
may  become  more  or  less  obscured  and  modified  in  the  adult. 

£%  Differentiation  of  Organs  and  Tissues. — We  have  al- 
ready in  these  three  layers  and  their  foldings  the  fundamental 
outline  of  that  differentiation  which  is  to  give  us  the  com- 
plex animal  form  found  in  the  adult.  From  these  layers, 
singly  or  in  combination,  all  the  tissues  and  organs  of  the 
body  arise.  The  various  layers  become  locally  thickened, 
folded,  or  otherwise  modified  in  form  by  rapid  cell  division, 
thus  producing  the  beginnings  of  organs.  At  a  later  date 
differentiation  takes  place  among  the  cells,  and  tissues  arise 
(see  next  chapter).  In  general  each  layer  gives  rise  to  such 
structures  as  its  position  and  relation  to  the  outer  layers  would 
suggest.  This  is  especially  noticeable  in  the  ectoderm  and  en- 
toderm.  The  former  is  more  closely  related  to  the  outside 
world,  and  from  it  are  produced  the  protective  and  sensory 
structures.  These  include  the  outer  portion  of  the  skin  and 
the  hard  parts  often  associated  with  it,  and  the  whole  nervous 
system  together  with  the  sensitive  portions  of  the  organs  of 
special  sense.  The  entoderm  is  derived  jirpm  the  cells  which 
contain,  or  at  least  are  closely  related  to,  the  food  originally 
stored  in  the  ovum  (Fig.  n),  and  it  comes  to  lie  in  the  in- 
terior of  the  embryo.  It  furnishes  the  lining  of  the  adult 
digestive  tract  as  well  as  the  essential  parts  of  the  glands  aris- 
ing from  it.  The  mesoderm  gives  origin  to  the  muscles  and 
to  the  supportive  or  skeletal  structures  generally.  Many  of 
the  organs  are  made  up  of  contributions  from  two  or  all  of 
these  germinal  layers.  Students  must  be  referred  to  special 
textbooks  on  embryology  for  a  more  extended  account  of  the 
manner  in  which  the  germinal  layers  give  rise  to  adult  organs. 


38  ZOOLOGY. 

58.  Summary. 

1.  All  the  higher  animals  begin  life  as  a  single  cell  and 
reach  their  adult  condition  by  a  continuous  series  of  divisions. 
By  the  growth  and  specialization  of  the  cells  arising  from 
these  divisions  the  great  complexity  of  the  adult  body  is  pro- 
duced. 

2.  This    initial   cell — the    fertilized    ovum — represents    the 
fusion  of  two  independent  and  unlike  cells:  the  ovum   (fe- 
male) and  the  spermatozoon  (male). 

3.  Before  the  union   (fertilization)   occurs,  the  ovum  re- 
duces its  nuclear  material,  by  two  successive  divisions,  to  one- 
fourth  its  original  amount  and  the  chromosomes  to  one-half 
their  original  number,  without  a  corresponding  reduction  of 
the  cytoplasm.     The  spermatozoon  in  its  development  seems 
to  undergo  a  similar  reduction  of  chromosomes. 

4.  After  the  union  of  the  male  and  female  cells  the  fertil- 
ized ovum  divides  rapidly  (segmentation  or  cleavage)   form- 
ing a  mass  of  cohering  cells.     The  nature  of  these  cells  and 
of  the  mass  depends  much  on  the  amount  of  yolk  in  the  ovum 
and  on  its  distribution. 

5.  By  processes  which  differ  in  different  animals  accord- 
ing to  the  nature  of  the  segmentation,  the  cells  become  ar- 
ranged with  a  layer  outside  (ectoderm),  a  layer  within  (ento- 
derm),  and  from  these  a  third  layer  or  mass  of  cells  lying  be- 
tween the  other  two  (mesoderm).     The  entoderm  bounds  a 
cavity  (archenteron)  which  communicates  by  a  pore  (blasto- 
pore)   with  the  outside  world.     Within  the  mesoderm  may 
be  found  a  cavity  (ccelom). 

6.  The  ectoderm  gives  rise  to  the  outer  portions  of  the 
skin,  its  protective  and  sensory  structures,  to  the  nervous  sys- 
tem, and  frequently  to  the  lining  of  the  openings  into  the 
body.     The  entoderm  lines  the  principal  part  of  the  digestive 
tract.     The  mesoderm  gives  rise  to  most  of  the  other  struct- 
ures of  the  body. 

59.  Suggestive  Topics  for  Library  Work. 

I.  What  suggestions  have  been  offered  as  to  the  advantage 


FROM    SIMPLE    CELL    TO    COMPLEX    ANIMAL.  39 

of  the  addition  of  the  male  nucleus  to  that  of  the  female  in 
fertilization?  Has  a  similar  result  ever  been  attained  arti- 
ficially by  means  of  chemical  or  other  stimuli? 

2.  What  suggestions  have  been  offered  as  to  the  signifi- 
cance of  the  process  of  maturation?     Trace  the  maturation 
of  the  sperm  cells  more  fully. 

3.  What    classification    of    ova    do    the    textbooks    make? 
What  is  the  basis  of  the  classification?     To  what  extent  do 
eggs  of  different  animals  vary  in  size,  shape,  envelopes,  etc.  ? 
Give  examples. 

4.  Is  there  any  explanation  of  the  fact  that  there  is  such 
a  difference  in  the  amount  of  food  substance  in  the  eggs  of 
different  animals? 

5.  Trace  out  by  reference  to  a  textbook  of  embryology  the 
principal  changes  by  which  the  adult  digestive  tract  is  derived 
from  the  simple  condition  found  in  the  gastrula   (archente- 
ron).     What  is  the  fate  of  the  blastopore?     How  does  the 
permanent  mouth  originate? 

60.  Exercises  for  the  Laboratory. 

The  teacher  should  secure  demonstrations  of  some  of  the 
smaller  ova  (as  of  the  snail,  fish,  sea-urchin y  for  examina- 
tion with  the  microscope.  Compare  the  ovum  taken  from  the 
ovary  of  a  hen  with  a  new  laid  egg,  noting  especially  the 
structure  of  the  latter.  Obtain  spermatozoa  from  the  testis 
of  a  recently  killed  animal  (as  mouse,  fowl,  etc.)  and  ex- 
amine with  highest  powers  of  the  microscope.  If  possible 
secure  permanent  mounts  of  segmenting  eggs  of  sea-urchins, 
showing  the  2,  4,  8-celled  stages. 


CHAPTER    V. 
CELLULAR  DIFFERENTIATION.— TISSUES. 

61.  Two  things  of  importance  happen  as  the  organism  de- 
velops from  the  simple  condition  of  the  ovum  to  the  great 
complexity  of  structure  in  the  adult:  (i)  the  increase  in  the 
number  of  cells,  which  is  quantitative  in  nature,  and  (2)  the 
differentiation  of  cells,  whereby  the  cells  of  the  various  parts 
become  very  diverse  in  shape,  composition,  and  powers.     This 
is  a  qualitative  change.     It  is  not  yet  fully  known  how  much 
of  the  difference  in  the  cells  of  the  various  tissues  is  due  to 
qualitative  differences  in  the  daughter  cells  of  a  given  division, 
and  how  much  is  due  to  external  influences  and  the  interrela- 
tions of  the  cells  after  division.     We  know  that  gravity  act- 
ing on  the  food  substance  of  the  ovum  before  division  does 
produce   such   differences   among  the   daughter  cells   of   the 
early  cleavage  stages  as  lead  to  results  as  diverse  as  ectoderm 
and  entoderm.     On  the  other  hand,  it  has  been  shown  by  ex- 
periment that,  even  as  high  up  in  the  animal  scale  as  the 
lower  vertebrates,  the  blastomeres  of  the  two  or  four-celled 
stage  may  be  shaken  apart  and  each  develop  into  a  small  but 
perfect  embryo.     This  experiment  shows  that  up  to  this  stage 
no  specialization  has  taken  place  which  limits  the  products 
that  come  from  these  cells.     The  blastomeres  do  not  so  de- 
velop after  the  8  or  i6-celled  stage  is  reached,  so  far  as  is 
known.     We  are  ignorant  of  the  causes  which  determine  that 
one  cell  shall  develop  into  a  muscle  cell  and  its  neighbor  into 
a  bone  cell. 

62.  Tissues. — A  tissue  is  to  be  defined  as  a  group  of  similar 
cells  suited  by  their  differentiation  to  the  performance  of  a 
definite  function.     This  differentiation  affects  the  size,  shape, 
and  the  interrelations  of  cells,  and  likewise  the  chemical  and 
physical  structure  of  the  protoplasm,  in  such  a  manner  as  to 

40 


CELLULAR    DIFFERENTIATION.  4! 

cause  great  variation  in  their  powers  and  activities.  The 
chemical  differences  are  especially  shown  in  excretion  and  se- 
cretion whereby  various  sorts  of  materials  are  deposited  with- 
in and  between  the  cells  of  the  different  tissues.  The  ma- 
terial deposited  between  the  cells  is  known  as  intercellular  sub- 
stance. The  intercellular  substance  differs  much  in  character 
and  amount.  Both  the  cells  and  the  intercellular  substance 
are  important  in  enabling  the  tissue  to  perform  its  work.  In 
general  if  the  tissue  is  active  (as  muscle)  the  cellular  differ- 
entiation is  the  important  point;  if,  however,  the  function  is 
a  more  passive  one,  as  support  or  protection,  the  nature  of 
the  intercellular  substance  rather  than  the  cells  determines  its 
character  (bone,  connective  tissue). 

63.  Classification  of  Tissues. — From  a  physiological  point 
of  view  tissues  may  be  classed  in  one  of  two  groups :  vegeta- 
tive, and  active.     The  vegetative  tissues  are  those  which  per- 
form the  more  passive  functions,  as  nutrition,  protection,  sup- 
port, etc.     They  resemble  the  plant  tissues  in  their  functions. 
The  two  chief  classes  of  vegetative  tissues  are:  epithelial  or 
bounding  tissues,  and  supportive  or  connective  tissues.     The 
active  tissues  may  be  looked  upon  as  the  characteristic  tissues 
of  animals.     The  muscular  and  nervous  tissues  belong  to  this 
group. 

64.  Epithelial  Tissue. — This  tissue  is  characterized  by  its 
•'primitive  form,  i.  e.,  by  its  relative  lack  of  differentiation,  by 

the  fact  that  it  is  the  first  to  appear  in  individual  development 
(ectoderm  and  entoderm  in  the  gastrula),  and  by  the  absence 
of  intercellular  substance.  It  is  a  bounding  tissue  and  consists 
typically  of  a  single  layer  of  cells,  although  several  layers  may 
occur.  Epithelium  bounds,  by  its  own  cells  or  their  products, 
the  outside  of  the  body,  the  lumen  of  the  digestive  tract  and 
its  outgrowths,  as  well  as  the  body  cavity  and  the  structures 
contained  in  il 


65.  Kinds  of  Epithelial  Tissue. — Located  in  a  position 
superficial  to  the  other  tissues,  epithelium  is  subject  to  a  wide 


4.2  ZOOLOGY. 

range  of  variation  both  as  to  form  and  function.  Besides  its 
primary  work  as  a  protective  layer,  the  epithelium  may  have 
a  glandular  function,  being  favorably  situated  for  the  final 
elimination  of  products  from  the  body.  Since  it  is  especially 
exposed  it  is  the  layer  best  adapted  by  position  to  receive  those 
external  stimuli  which  we  know  to  play  such  an  important  role 
in  the  life  of  all  organisms.  The  position  of  the  epithelium 
also  renders  it  specially  liable  to  destruction.  To  compensate 
for  this  its  primitive  or  undifferentiated  character  makes  it 
particularly  capable  of  regenerating  portions  of  itself  which 
may  have  been  lost.  Epithelium  is  often  especially  active  also 
in  the  regeneration  of  other  than  simple  epithelial  structures. 
In  close  connection  with  this  latter  regenerative  quality  is  to 
be  considered  the  fact  that  epithelium  gives  rise  to  the  re- 
productive or  sexual  cells  by  which  new  individuals  are  pro- 
duced. The  foregoing  enumeration  of  functions  suggests  the 
physiological  classification  of  epithelia: — bounding,  glandular, 
sensory,  and  reproductive  epithelia.  The  same  layer  may 
fulfill  several  of  these  functions  at  once. 

66.  Bounding   Epithelium. — The   ordinary  protective   epithelium   may 
be  made  up  of  cells  cuboidal  in  shape  (Fig.  14,  5),  or  columnar  (Fig.  14, 
A),  or  much  flattened   (Fig.  14,  C).     In  extreme  cases  of , flattening  and 
hardening  we  have  squamous  epithelium,  e.  g.  the  outer  cells  of  the  human 
epidermis.     Motile   protoplasmic   projections   often   extend    from   the   free 
surface  of  the  epithelium.    Flagellate  epithelium  (Fig.  77,  D}  has  one  such 
projection  from  each  cell,  whereas  dilate  epithelium  (Fig.  14,  D,  E)  has 
numerous   small  ones.     Cilia   aje  more  common   in  the   lower  groups   of 
animals,  but  are  found,  even  in  mammals,  in  the  moist  internal  passages,  as 
in  the  nose,  trache^,/ etp. 

Membranes  bounding*  the  body  cavity  are  called  scrolls  membrane 
(endothelium) .  TCtieJ  liSjmg  of  the  digestive  tract  is  described  as  a  mucous 
membrane.  \tj 

Epithelial  cells  orten  secrete  upon  their  outer  surface  a  layer  of  mate- 
rial (cuticula),  which  serves  to  protect  the  cells  beneath  and  the  organism 
as  a  whole  from  external  influences  (as  the  covering  of  the  cray-fish). 
From  the  epithelium  arise  various  outgrowths,  as  scales,  hair,  feathers, 
and  the  like. 

67.  Glandular  Epithelium. — The  ordinary  columnar  or  pavement  epi- 
thelium may  here  and  there  present  cells  or  areas  of  cells  which  are  spe- 
cially active  in  producing  and  pouring  out  on  their  free  surface  certain 


CELLULAR    DIFFERENTIATION. 


43 


materials,  called  secretions.     In  its  simplest  form  the  gland  or  secreting 
surface  may  consist  of  a  single  cell,  as  the  goblet  or  slime  cells  (Fig.  15,  a). 


FIG.  14.  Various  kinds  of  epithelial  cells  (semi-diagrammatic).  A,  columnar;  B, 
cuboidal;  C,  pavement;  D  and  E,  ciliate  (sectional  views).  In  F  is  shown  the  surface 
view  of  pavement  epithelium,  cl.,  cilia;  cu.,  cuticula. 

Questions  on  the  figure. — For  what  different  uses  would  you  judge 
these  variously  shaped  epithelial  cells  to  be  suited?  Under  what  circum- 
stances and  on  what  surfaces  would  you  expect  to  find  each  type?  Com- 
pare with  your  reference  texts  and  see  if  they  are.  so  found.  Under  what 
circumstances  is  a  cuticula  to  be  expected?  Where  would  it  be  a  disad- 
vantage? What  are  cilia? 

FIG.  15. 

,-a 


\ 


I 


FIG.  15.  Glandular  Epithelium,  a,  goblet  or  slime  cells, — unicellular  glands;  b, 
similar  cells  which  have  become  depressed  below  the  surface,  and  empty  their  secretion 
through  a  duct. 

Questions  on  the  figure.— Are  the  glandular  cells  modified  epithelial 
cells?    In  what  respects  do  they  differ  from  the  cells  about  them? 


44 


ZOOLOGY. 


Such  a  cell  may  become  much  enlarged  and  sink  below  the  general 
level  of  the  epithelium,  retaining  in  the  meantime  a  narrow  connection 
with  the  exterior  (Fig.  15,  b).  Multicellular  glands  represent  areas  of  such 
cells  which  have  sunk  below  the  surrounding  surface,  forming  a  tube-  or 
flask-shaped  cavity,  which  may  become  very  much  branched.  Glands  with 
such  branched  ducts  are  described  as  compound.  They  consist  of  numer- 
ous final  secretory  sacs  communicating  by  ductules  with  a  common  duct 
or  outlet  to  the  surface.  Transitional  conditions  between  the  simple 
secretory  epithelium  and  the  compound  gland  may  be  seen  in  Fig.  16. 

68.  Sensory  Epithelium. — In  the  lower  animals  there  may  be  found 
here  and  there  over  the  surface  of  the  body  modified  epithelial  cells,  which 

FIG.  1 6. 


FIG.  16.  A  series  of  diagrams  showing  progressive  stages  in  the  development  of  a 
njulticellular  gland  from  an  area  of  glandular  epithelial  cells.  C  and  D  show  two  some- 
what different  types  of  gland, — the  cup-shaped  and  the  tubular,  e,  bounding  epithe- 
lium; g,  gland  cells;  d,  duct;  c,  connective  tissue. 

Questions  on  the  figure. — How  do  the  compound  glands  seem  to  arise 
from  the  simpler  condition?  What  is  the  evidence  that  glands  are  lined 
throughout  with  epithelium?  What  is  gained  in  the  sinking  of  the  glands 
below  the  surface? 

are  specially  capable  of  being  stimulated  by  contact  or  other  stimuli  to 
which  the  organism  may  be  exposed.  Likewise  in  higher  forms  we  find 
highly  specialized  areas  of  sensitive  cells,  which  can  be  shown  to  belong 
primarily  to  the  epithelium.  These  are  the  end  organs  of  special  sense, 
as  touch,  sight,  and  the  like,  and  they  get  their  special  value  from  their 


CELLULAR    DIFFERENTIATION. 


45 


connection  with  what  will  be  described  presently  as  the  nervous  tissues  of 
the   central   nervous   system.     The   sensory  cells   are   typically  thread-like 


FIG.  17.  Sensory  and  muscular  epithelium.  A,  sensory  epithelium,  from  Worm, 
showing  some  of  the  epithelial  cells  (e)  modified  into  sensory  cells  (s).  B,  epithelial 
cells  from  Hydra  showing  contractile  or  muscular  processes  at  base  (w). 

Questions  on  the  figure. — Is  there  anything  to  suggest  that  the  sen- 
sory cells  are  modified  epithelial  cells?  What  are  the  principal  changes 
which  they  have  undergone  as  compared  with  the  unmodified  epithelium? 

FIG.  18. 


FIG.  1 8.  Diagram  of  a  portion  of  the  ovary  of  Sea-urchin  showing  the  eggs  arising 
from  the  epithelium  (reproductive  epithelium)  by  constriction,  e,  epithelium;  o,  ova 
in  different  stages  of  growth. 

Questions  on  the  figure.— What  is  an  ovary  in  its  simplest  form?  Is 
the  reproductive  epithelium  ectodermal,  entodermal,  or  mesodermal  in 
origin,  as  a  rule? 

or  hair-like  in  form,  often  extended  as  fine  fibres  at  the  inner  end,  whereby 
connection  is  established  with  the  nerves  (Fig.  17,  A). 


^6  ZOOLOGY. 

69.  Reproductive  Epithelium.— The  sexual  cells,  both  male  and  female, 
are  developed  from  epithelium,  -ectodermal,  entodermal,  or,  as  is  usually 
the  case,  mesodermal.  The  budding  of  the  sexual  epithelium,  in  the  devel- 
opment of  the  germ  cells  suggests  the  formation  of  glands  (Figs.  18,  19). 
The  sexual  cells  often  develop  at  the  expense  of  the  epithelial  cells  about 

them. 

FIG.  19. 


o 

FIG.  19.  Section  through  ovary  of  a  young  Mammal  (modified  from  Wieder- 
sheim).  The  eggs  (o)  are  seen  to  be  formed  from  the  epithelium  by  a  process  some- 
what more  complex  than  in  Fig.  18.  c,  connective  tissue  of  ovary;  e,  epithelium;  f, 
follicle  of  epithelial  cells  in  which  the  ova  ripen;  o,  ova  in  different  stages  of 
ripeness. 

Questions  on  the  figure. — In  the  ovary  of  the  mammal  what  addi- 
tional service  does  the  epithelial  layer  render  the  ovum  after  its  forma- 
tion? Is  it  apparent  that  there  is  anything  gained  by  the  sinking  of  the 
ovarian  follicles  into  the  tissue  of  the  ovary,  instead  of  escaping  imme- 
diately, as  in  Fig.  18? 

70.  Supportive  or  Connective  Tissues. — This  class  of 
tissues  embraces  the  bulk  of  the  non-active  tissues  in  animals. 
They  vary  much  in  appearance  and  structure,  agreeing  in  little 
except  in  their  mesodermic  origin,  their  passivity,  and  in  the 
prevalence  of  intercellular  substance.  The  intercellular  sub- 
stance gives  the  distinctive  character  to  the  connective 
tissues,  the  cells  having  a  relatively  unimportant  place 
after  the  production  of  the  intercellular  substance.  The 
general  function  of  the  supportive  tissues  is  to  bind  and 
sustain  the  more  active  tissues  in  their  relations  to  the 
body  as  a  whole.  The  classification  of  supportive  tissues 


CELLULAR    DIFFERENTIATION. 


47 


is  based  on  differences  in  the  intercellular  substance.  This 
may  be  fluid  (as  in  blood)  or  solid  (as  in  bone)  ;  it  may  be 
homogeneous  (as  in  some  forms  of  cartilage),  or  fibrous;  it 

FIG.  20. 


FIG.  20.     Cellular    Connective    Tissue,    showing   large   vacuoles,    v,    in    the   protoplasm. 

Questions  on  the  figure. — Would  you  say  that  these  cells  are  of  a 
high  or  a  low  order  of  differentiation?  Why?  Is  there  any  intercellular 
substance?  Where  is  tissue  of  this  kind  found?  (See  reference  texts.) 

FIG.  21. 


FIG.  21.     Gelatinous    connective    tissue,    showing    stellate    cells     (c),    epithelium     (e), 
the    gelatinous    intercellular    substance    (s),    and    the    intercellular    fibres    (f). 

Questions  on  the  figure.— What  seems  to  be  the  relation  of  the  epi- 
thelial layer  to  the  tissue  below  it?  What  classes  of  cells  are  found  in 
the  gelatinous  tissue?  What  is  their  origin?  What  is  the  nature  of  the 
intercellular  substance?  Are  the  fibres  cellular  or  intercellular? 

may  be  almost  wholly  organic,  or  very  largely  inorganic.  The 
principal  classes  are  cellular  connective  tissues,  gelatinous  con- 
nective tissue,  fibrous  connective  tissue,  cartilaginous  tissue, 
and  osseous  tissue. 


48  ZOOLOGY. 

71.  Cellular  or  Vesicular  Tissue  forms  an  exception  to  the  general 
rule   of   abundant   intercellular   substance.     It   is    an   embryonic   tissue, — a 
forerunner  of  the  more  permanent  tissues, — and  is  chiefly  interesting  from 
that  fact.     The  cells  have  large  vacuoles  or  vesicles  which  are  enveloped 
by  a  thin  layer  of  protoplasm    (Fig.   20).     It  is  found  in  the  notochord 
of  vertebrates. 

72.  Gelatinous  tissue  has  a  matrix  of  intercellular  substance  envelop- 
ing stellate  cells,  the  radiating  projections  of  which  serve  to  connect  them 
Fibres  are  often  developed  in  the  matrix.     This  tissue  is  abundantly  found 
in  the  jelly-fish   (see  Fig.  21). 

73.  Fibrous  connective  tissue  has  in  its  ground  substance  a  rich  supply 
of  fibrils  variously  arranged.     The  cells  or  corpuscles  are  often  elongated 
and  branched.     If  the  intercellular  fibres  cross,  running  in  various  direc- 
tions, a  loose  yielding  tissue  results,  as  in  the  ordinary  connective  tissue 
about  the  muscles  and  nerves   (Fig  22,  A}  ;  if  the  fibres  are  parallel  the 
tissue  naturally  becomes  more  compact.     There  are  two  types  of  the  more 
compact  sort  differing  in  the  quality  of  the  fibres.     The  latter  may  be  white 
and  inelastic,  as  in  tendons,  or  yellow  and  elastic.     Fat  is  frequently  de- 
posited as  spherical  drops  of  oil   (Fig.  22,  J5)   in  the  cells  of  connective 
tissue. 

FIG.  22. 

A 


FIG.  22.  Fibrous  connective  tissues.  A,  ordinary  connective  tissue  found  binding 
muscle  and  nerve  fibres,  showing  the  fibrous  intercellular  substance.  The  cells  (c)  are 
never  conspicuous  in  this  tissue.  B,  adipose  connective  tissue  showing  fat-laden  cells 
among  the  fibres  (/).  o,  oil  droplets  in  the  cells. 

Questions  on  the  figure. — In  these  two  types  of  tissue  which  element 
gives  special  character  to  the  tissue,  the  cells  or  the  intercellular  substance  ? 
How  would  the  deposition  of  large  drops  of  oil  in  the  cell  affect  the  activity 
of  the  cell?  Why?  Why  are  fatty  deposits  less  hurtful  amid  connective 
tissue  than  elsewhere  in  the  body? 
» 

74.  Cartilage. — In  cartilage  the  intercellular  matrix  is  much  firmer 
than  in  those  tissues  already  described.  It  may  appear  homogeneous  as 
in  rib  cartilage  (Fig.  23,  A}  ;  or  it  may  contain  numerous  fibres  which  give 
coherence  and  elasticity.  The  cells. are  usually  rounded  except  where  they 
have  been  flattened  by  mutual  pressure,  and  usually  occur  in  pockets  in 


CELLULAR    DIFFERENTIATION. 


49 


the  matrix.  Cartilage  is  bounded  on  its  free  surfaces  by  a  fibrous  mem- 
brane, the  pcrichondrium.  This  membrane  assists  in  the  growth  of  the 
cartilage.  There  are  no  blood  capillaries  in  cartilage. 

Salts  of  lime  may  be  deposited  in  the  intercellular  substance,  giving  it 
some  of  the  qualities  of  bone. 

FIG.  23. 


FIG.  23.  Cartilage.  A,  Hyaline  cartilage;  B,  fibrous  cartilage.  In  the  latter  a  large 
portion  of  the  intercellular  substance  is  conspicuously  fibrous.  The  cells  occur  in 
pockets  (/>)  in  the  matrix;  f,  intercellular  fibres. 

Questions  on  the  figure. — What  are  the  points  of  similarity  and, of 
difference  in  the  two  types  of  cartilage?  In  what  manner  do  the  multi- 
cellular  pockets  arise?  What  is  the  nature  and  origin  of  the  intercellular 
substance  in  each  case? 


75.  Osseous  or  Bony  Tissue. — These  tissues  are  found  only  in  verte- 
brates, and  are  the  most  complicated  of  the  supportive  tissues.  The  firm 
matrix  which  is  secreted  by  the  bone  cells  consists  of  a  mixture  of  organic 
substance  and  inorganic  matter,  especially  the  salts  of  lime.  The  cells 
with  their  fine  filamentous  branches  occur  more  or  less  regularly  between 
thin  plates  or  lamella  of  the  bony  material.  A  cross-section  of  one  of  the 
long  bones  shows  the  typical  condition.  The  periosteum  is  a  superficial 
fibrous  membrane  about  the  bone,  well  supplied  with  blood  vessels.  Its 
inner  layer  of  cells  is  capable  of  producing  bone.  Within  this  is  a  region 
of  firm  bone,  in  which  a  series  of  lamellae  are  parallel  with  the  surface 
of  the  periosteum.  Between  the  lamellae  occur  the  spaces  (lacunae}  occu- 
pied by  the  bone-cells  which  have  been  left  behind  as  the  matrix  was 
deposited.  Deeper  in  the  bone  the  lamellae  and  cells  are  in  concentric 
layers  about  the  numerous  blood  vessels  (occupying  spaces  known  as 
Haversian  canals']  which  penetrate  the  bone,  chiefly  in  a  longitudinal 
direction.  The  included  bone-cells  communicate  with  each  other  and  with 
the  blood  vessels  by  processes  which  occupy  minute  canals  (canaliculi) 
5 


50  ZOOLOGY. 

in  the  intercellular  substance  (Fig.  24).  Within  this  region  and  imme- 
diately surrounding  the  central  cavity  of  the  bone  is  often  a  mass  of  spongy 
bone  in  which  the  regularity  of  arrangement  of  the  cells  is  lost.  Bone  may 
be  formed  by  replacing  cartilage,  or  wholly  independent  of  it. 

FIG.  24. 


FIG.  24.  Bony  Tissue.  A,  portion  of  cross-section  of  a  bone,  the  upper  portion  of 
the  figure  representing  the  outer  surface  of  the  bone,  just  beneath  the  periosteum.  The 
open  spaces,  h,  are  Haversian  canals;  I,  lacuna,  occupied  in  life  by  bone  cells.  The 
minute  canals  through  the  bone  connecting  the  lacunae  are  canaliculi.  B,  a  portion 
of  one  Haversian  system  much  magnified,  h,  Haversian  canal,  containing  artery  (a), 
vein  (f),  lymphatic  spaces,  nutritive  cells;  c,  canaliculi;  /,  lacunae;  la,  plate  of  bony 
intercellular  substance. 

Questions  on  the  figure. — How  does  bone  compare  in  appearance  and 
structure  with  the  other  supportive  tissues?  How  is  its  intercellular  sub- 
stance laid  down?  How  are  the  cells  in  the  bone  nourished?  How  do 
they  come  to  lie  in  the  solid  bone?  What  changes  occur  in  this  type  of 
tissue  with  age?  What  is  the  function  of  the  Haversian  canal? 

Dentine  and  enamel,  though  differing  in  structure  from  bone,  are  to 
be  looked  upon  as  belonging  to  the  same  class  of  tissues.  They  differ 
chiefly  in  the  fact  that  no  cellular  elements  are  included  in  the  secretion. 
They  are  thus  harder  and  denser  than  bone. 

76.  We  find  all  stages  of  transition  between  the  more  sim- 
ple and  more  complex  supportive  tissues,  and  it  may  be  seen 
furthermore  that  there  is  a  fundamental  embryological 
sequence.  In  the  development  of  the  organism  the  simpler 
connective  tissues  give  place,  by  transformation  or  substitu- 
tion, to  the  more  complex.  The  cellular  connective  tissue  of 
early  life  is  replaced,  for  example,  by  cartilage,  and  this  may 
be  transformed  into  bone  in  adult  life. 


CELLULAR    DIFFERENTIATION.  5  3 

77.  Nutritive  Fluids. — The  body  fluids  known  as  blood  and  lymph  are 
frequently  classed  among  the  supporting  tissues,  the  fluid  portion  being 
regarded  as  the  intercellular  substance  and  the  corpuscles  as  the  cells. 


FIG.  25.     Blood  corpuscles  (amphibian),     c,  colored  corpuscles,  flatwise  and  in  profile; 
/,    colorless   corpuscles    (.leucocytes). 


FIG.  26. 


FIG.  26. 


Blood  corpuscles   (human),     c,  colored;  /,  leucocytes.     The  red  cells  tend  to 
collect  in  rows  with  the  sides  in  contact. 


Questions  on  figures  25  and  26. — Compare — by  means  of  the  figures, 
the  text  and  reference  books — the  colored  and  colorless  corpuscles  of 
these  two  types  of  vertebrates  and  note  the  differences.  In  what  other 
respects  do  the  colored  cells  differ  from  the  white?  Which  are  the  less 
highly  differentiated?  Reasons  for  your  view?  Why  are  the  colorless 
corpuscles  also  called  phagocytes? 

They  differ  however  from  the  ordinary  tissues  in  the  important  fact  that 
the  intercellular  substance  is  not  produced  by  the  cells.  In  the  vertebrates 
these  cells  are  of  two  kinds,  the  amoeboid  or  colorless  and  the  colored. 
Both  kinds  occur  in  the  blood ;  the  colorless  alone  are  found  in  the  lymph. 
The  colored  corpuscles  are  relatively  numerous  and  are  disc  shaped.  Re- 
garded as  cells  they  present  a  series  of  degenerative  changes  which  results 
in  a  loss  of  the  distinctively  protoplasmic  character,  by  the  substitution 


5- 


ZOOLOGY. 


of  certain  proteid  substances,  one  of  which — haemoglobin — is  notable  for 
its  affinity  for  oxygen.  The  degeneracy  may  go  to  the  extent  of  the  entire 
loss  of  the  nucleus,  as  in  the  mammals.  The  colorless  cells  have  the 
power  of  independent  motion  such  as  is  found  in  the  amoeba,  and  may  ingest 
solid  particles  of  food.  The  body-fluids  of  the  invertebrates  contain  as  a 
rule  only  colorless  corpuscles,  and  are  therefore  more  like  the  lymph  of 
the  vertebrates.  When  their  blood  is  colored  it  is  usually  from  pigment 
in  the  plasma  or  fluid  portion  of  the  blood.  In  addition  to  the  cells  the 
blood  carries  a  rich  supply  of  proteid  and  other  substances  for  use  in  the 
tissues,  and  of  waste  products  in  process  of  removal  from  the  body. 

78.  Muscular  Tissue. — The  remaining  tissues  are  charac- 
teristically active.  Muscular  tissue  by  its  contractility  has  the 
power  of  producing  movements  of  the  parts  to  which  it  is 
attached.  This  contractility  of  muscle  may  be  looked  upon 

FIG.  27. 


FIG.  27.     Plain  muscle  fibres,     n,  nucleus  of  muscle  cell;  p,  undifferentiated  cell  proto- 
plasm; p',  the  differentiated  contractile  portion  of  the  cell. 

Questions  on  the  figure. — What  are  the  two  principal  portions  of  these 
cells?  How  do  very  young  muscle  cells  compare  with  older  ones  in  the 
relative  amount  of  these  portions  in  the  cell?  Which  is  the  more  highly 
differentiated  portion?  Where  are  such  tissues  found  in  the  animal  body? 
Why  are  muscle  fibres  elongated? 


CELLULAR    DIFFERENTIATION.  53 

as  a  specialization,  and  limitation  in  direction,  of  the  power  of 
contraction  which  we  have  seen  to  be  resident  in  all  living 
protoplasm.  Muscular  tissue  differs  somewhat  in  structure 
and  degree  of  differentiation  in  various  animals,  but  in  general 
agrees  in  the  presence  of  elongated  fibres  which  are  to  be 
considered  as  modified  cells  or  parts  of  cells.  The  contractile 
muscle  substance  is,  in  part  at  least,  a  plasmic  product  rather 
than  mere  protoplasm;  yet  it  differs  from  the  plasmic  inter- 
cellular substance  of  the  tissues  already  described  in  that  it  is 
deposited  within  rather  than  among  the  cells.  Two  stages 
in  the  differentiation  of  muscular  substance  are  to  be  noted : 
( i )  the  fibres  may  be  plain,  in  which  case  we  find  elongated, 
contractile  single  cells  without  conspicuous  external  differen- 
tiation (Fig.  27)  ;  (2)  cross-striate  fibres,  which  always  show 
conspicuous  differentiation  of  parts  in  each  fibre  as  seen  under 
the  microscope.  The  plain  fibres  are  characteristic  of  slug- 
gish animals,  and  those  parts  of  animals  whose  muscular 
action  is  least  prompt  in  response  to  the  nervous  stimuli  (e.  g., 
digestive  tract  in  vertebrates).  The  cross-striated  fibre  usu- 
ally represents  several  incompletely  separated  cells,  or  a  multi- 
nucleate  condition  of  a  much-grown  and  metamorphosed  single 
cell.  In  both  classes  the  fibres  are  made  upT  of  numerous 
minute  strands  or  fibrillce  which  in  the  plain  muscle  are  homo- 
geneous throughout,  but  in  the  cross-striated  are  made  up  of 
alternating  segments  of  lighter  and  darker  optical  appear- 
ance (Fig.  28,  B).  The  undifferentiated  protoplasmic  rem- 
nant is  often  very  small  in  amount,  and  is  collected  about  the 
nucleus  (Figs.  27,  28).  It  may  be  at  the  surface  of  the  fibre 
or  in  the  centre,  enveloped  by  the  contractile  matter.  A  thin 
membrane  (sarcolemma)  binds  the  fibrillae  into  fibres.  The 
fibres  are  bound  together  by  strands  of  connective  tissue  into 
bundles,  and  of  these  bundles  the  muscle  is  made  up. 

79.  Origin  of  Muscle  Tissue. — In  those  animals  in  which  a  true  meso- 
derm  is  wanting,  the  epithelial  cells  may  develop, ,  at  their  inner  extremity, 
contractile  roots,  either  plain  or  striate  (Hydra,  Fig.  17,  B}.  These  cells 
may  wholly  lose  their  epithelial  quality  and  position  and  become  entirely 
muscular.  In  the  higher  animals  this  is  very  much  modified  by  the  early 


54 


FIG.  28.  Diagram  of  nervous  and  cross-striate  muscular  tissue,  showing  the  mode  of 
connection  between  nerve  fibres  and  muscle  fibres.  A,  nerve  cell  (g)  connected  with 
muscle  fibre  (mf.)  by  nerve  fibre  (n./.).  The  muscle  fibre  (w.f.)  is  composed  of 
numerous  fibrils  (/)  which  are  made  up  lengthwise  of  alternating  discs  of  lighter  and 
darker  substance.  These  fibrils  are  shown  more  highly  magnified  in  B  and  C.  In 
B  the  fibril  is  uncontracted;  in  C  it  is  contracted.  D,  nerve  fibre  more  highly 
magnified  showing  a,  axis;  m,  medullary  sheath;  and  s,  Schwann's  sheath;  ax.,  axon; 
d,  dendron;  n,  node;  n.m.,  nerve-muscle  plate. 


CELLULAR    DIFFERENTIATION.  55 

appearance  of  separate  mesoderm  from  which  the  whole  muscular  system 
is  derived. 

80.  Nervous  Tissue:  its  Functions. — The  nervous  tissues 
are  in  close  relation  on  the  one  hand  to  the  sensory  epithelium 
and  on  the  other  to  the  muscular  tissue.     Through  the  former 
they  receive  the  stimuli  from  the  outside  world;  by  means  of 
their  connection  with  the  latter  they  are  enabled  to  effect 
responses.     The  reception  of  stimuli,  the  transmission  of  the 
results  of  stimulation,  and  the  initiation  of  appropriate  re- 
sponses constitute  the   fundamental  work  of  nervous  tissue 
(Fig.  28,  A,  D).     In  some  of  the  lower  Metazoa  the  same  cell 
may  do  all  these  tasks. 

81.  Structure. — The  principal  elements  of  nervous  tissue 
are  the  nerve-cells    (ganglion-cells)    and   nerve- fibres.     The 
cells,  which  are  the  centres  of  nervous  activity,  are  usually 
large  with  conspicuous  nuclei.     The  fibres  are,  in  their  essen- 
tial parts  merely  outgrowths  of  the  ganglion-cells.    These  out- 
growths are  of  two  sorts :  the  dendron,  which  is  a  much,  and 
irregularly,  branched  structure ;  and  the  axon,  or  nervous  fibre 
proper.     Each  cell  may  have  one  or  more  processes  arising 
from  it.     These  fibres  may  pass  just  as  they -arise  from  the 
cells,  without  special  structural  modification,  to  their  connec- 
tions.    Such  are  called  non-medullated  or  gray  fibres.     There 
are  usually  however  one  or  more  protective  sheaths  formed 
about  this  essential  axis:  (i)  the  medullary  sheath,  consisting 
of  a  framework  filled  with  a  fatty  material,  surrounded  by 
(2)  Schwann's  sheath,  a  homogeneous  sheath  with  occasional 
nuclei  along  its  course  (Fig.  28,  D).     Fibres  possessing  the 
medullary  sheath  are  called  medullated  or  white  fibres. 

Questions  on  Fig.  28. — What  are  the  principal  points  of  contrast 
between  the  plain  and  the  cross-striate  muscular  fibres?  Enumerate  the 
principal  regions  of  the  nerve  cell  figured.  How  does  it  differ  from  a 
typical  cell  in  form?  What  are  the  principal  parts  of  the  nerve-fibre  (.D)  ? 
What  are  the  supposed  functions  of  these  various  portions?  Why  is  it 
necessary  for  nerve  cells  to  be  in  connection  with  other  kinds  of  cells? 
What  are  the  differences  between  the  contracted  and  uncontracted  muscle 
fibre  (B  and  C)  ?  What  is  meant  by  a  neurone? 


56  ZOOLOGY. 

A  nerve  cell  together  with  its  processes  is  called  a  neurone. 
The  whole  nervous  system  may  be  considered  as  made  up  of 
such  units,  which  connect  with  each  other  by  the  delicate 
terminal  branches  of  the  outgrowths.  See  Fig.  34. 

82.  Origin   of   Nervous   Tissue. — Nervous   tissue    always   arises   from 
the  ectoderm  of  the  embryo,  so  far  as  we  know.     In  some  of  the  lower 
forms   of  animals,   as  the  ccelenterata,   the  nervous  cells  may  be   derived 
individually   from   the   epithelium.     In   such   instances   they   have   a   close 
connection    with    those    muscle    elements    which    are    also    of    epithelial 
origin    (see  §79).     In  the  higher  forms  the  origin  of  the  nervous  mat- 
ter  from   the   ectoderm   is    somewhat   less    direct   but    essentially   similar. 
The    connection    of    the    nervous    centres    with    the    muscles    and    glands, 
etc.,   in   the   higher   animals    is   a    secondary   condition   and    is    the    result 
of   the   growth   of   the   nerve    fibres    toward    such    organs.      What   directs 
their  growth   to  the   right  place   when   the  fibres  begin  to  grow,   we  do 
not  know. 

83.  Summary. 

1.  The  individual  becomes  complex  by  the  increase  of  the 
number  of  cells,  and  by  their  differentiation. 

2.  A  tissue  consists  of  a  group  of  similar  cells  with  their 
products,  which  are  adapted  to  the  performance  of  special 
work  or  function. 

3.  Tissues  differ  morphologically  in  respect  to  the  form, 
arrangement,  and  structure  of  the  cells,  and  in  the  amount, 
arrangement  and  consistency  of  the  intercellular  substance. 

4.  Physiological   differentiation   accompanies   the   morpho- 
logical, the  division  of  labor  becoming  very  complete  in  the 
higher  forms.     The  physiological  value  of  a  tissue  may  de- 
pend either  upon  the  cells  or  the  intercellular  substance. 

5.  Tissues  may  be  classified  as  follows : 
A.  The  vegetative  or  passive  tissues. 

I.  Epithelial : — 

function : — protection,      absorption,      secretion, 

sensation,  reproduction,  etc. 
kind: — pavement,  columnar,  ciliate,  glandular, 

sensory,  muscular,  reproductive,  etc. 

II.  Supportive  or  connective: — 

function  : — binding,  support,  protection. 


CELLULAR    DIFFERENTIATION.  57 

character  : — abundant  intercellular  substance, 
form : — vesicular,  gelatinous,  fibrous,  cartilagi- 
nous, osseous-,  etc.   (blood  and  lymph). 
B.  The  active  tissues. 

III.  Muscular : — 

function : — irritability,  especially  to  nervous 
stimuli ;  contraction  in  a  definite  direction. 

form: — plain  and  cross-striate  (depending  on 
the  differentiation  of  the  contractile  sub- 
stance). 

IV.  Nervous : — 

function : — reception     of     general      stimuli, 

transmission    of    impulses,    interpretation, 

arid  the  initiation  of  appropriate  responses. 

form: — central  cells  (ganglion)  with  fibrous 

branches  (axon,  dendron). 

6.  The  epithelial  tissues  arise  from  ectoderm,  entoderm  and 
mesoderm ;  the  connective  tissue,  from  mesoderm ;  the  muscu- 
lar, chiefly  from  mesoderm;  and  the  nervous  tissue,  from 
ectoderm. 

84.  Exercises  for  the  Laboratory  (these  may  be  made  as 
extensive  as  time  and  facilities  will  allow). 

i.  Temporary  demonstrations  of  the  simpler  tissues  should 
be  made  by  the  teacher  or  pupil,  by  teasing  out  with  needles 
small  portions  of  the  appropriate  material  in  a  drop  of  water 
on  the  slide. 

(a)  Blood. — Compare  that  of  earth-worm  or  insect,  frog, 
man.     Place  a  drop  of  fresh  blood  on  the  slide  and  cover. 
Examine  at   once.      The   teacher   should   have   a  permanent 
preparation  of  the  blood  of  the  frog,  stained  to  show  nucleus 
of  corpuscles. 

(b)  Epithelium. — Mesentery  of  cat;  film  shed  from  skin 
of  frog  kept  a  few  days  in  captivity;  cells  scraped  from  the 
oesophagus  of  a  recently  killed  frog. 

(c)  Connective  Tissue. — Found  surrounding  muscle,  i.  e., 
lean  meat.    Compare  tendon. 


58  ZOOLOGY. 

(d)  Muscle. — From   wall  of   stomach,    from   heart,    from 
skeletal  muscles. 

(e)  Nerve  Fibres. — Small  portion  of  nerve  of  frog  or  cat. 
2.  The  teacher  should  secure  permanent  mounts  of  sections 

of  cartilage,  bone,  and  tooth  showing  dentine  and  enamel. 
Properly  stained  preparations  of  glandular  tissue,  of  nerve 
cells  and  their  branches,  and  of  reproductive  epithelium  (see 
appendix),  will  greatly  assist  the  pupils  in  securing  an  accurate 
idea  of  these  tissues  and  their  work. 


CHAPTER    VI. 

THE  GENERAL  ANIMAL  FUNCTIONS,  AND  THEIR 
APPROPRIATE  ORGANS. 

85.  Protoplasmic  Functions. — It  has  already  been  stated 
that  in  protoplasm  reside  the  fundamental  powers  belonging 
to  living  things.     Through  its  agency  all  the  metabolic  or 
assimilative  processes  are  performed.     Through  the  activity 
of  its  ferments  foods  undergo  changes  that  prepare  them  to 
be  used  in  the  manufacture  of  protoplasm  and  other  complex 
cell-substances.     In  protoplasm  occur  the  oxidation  and  other 
chemical  changes  which  result  in  the  manifestation  of  energy, 
as  heat,  motion,  light,  etc.,  accompanied  by  the  formation  of 
waste  products  which  are  to  be  eliminated.     The  power  of 
receiving  and  responding  to  surrounding  influences,  which  we 
have  called  irritability  and  contractility,  is  likewise  a  power 
of  protoplasm.     Out  of  these  arises  the  possibility  of  organ- 
isms becoming  adapted  to  their  surroundings.     It  is  not  yet 
possible  certainly  to  localize  all  these   functions  within  the 
individual  cell,  although  it  seems  probable  that  in  some  degree 
even  such  protoplasm  as  is  found  in  the  Amoeba  has  localized 
functions.     In  many  single-celled  animals  there  is  a  consider- 
able localization. 

86.  Division  of  Labor. — As  the  protoplasmic  units,  i.  e., 
the  cells,  increase  in  number  by  cell  division  and  form  large 
masses,  they  are  no  longer  subjected  to  the  same  influences, 
and  are  not  equally  favorably  situated  for  the  performance  of 
all  the  original  functions.    The  protoplasm  of  all  cells  retains 
the  power  of  using  food  and  of  building  up  its  own  substance, 
but  we  find  certain  activities  largely  given  over  to  special 
groups  of  cells ;  e.  g.,  secretion  is  specially  noticeable  in  some, 
contractility  in  some,  and  irritability  in  others.     This  division 
of  labor,  is  accompanied  by  a  corresponding  differentiation  of 

59 


60  ZOOLOGY. 

structure  which  constitutes  an  adaptation  to  the  special  work 
to  be  done,  and  is  of  great  advantage.  We  have  described 
these  structure-groups  as  tissues  (see  Chapter  V.). 

87.  Organs. — The  tissues  which  have  been  described  are 
never  independent,  but  are  associated  with  each  other  in  the  per- 
formance of  a  common  function,  to  form  an  organ.    In  each 
organ  there  is  usually  a  principal  tissue  which  determines  its 
function  (as  muscular  tissue  in  muscle,  or  the  glandular  tissue 
in  glands),  and  one  or  more  accessory  tissues  for  support  or 
control  (as  connective  or  nervous  tissue  in  the  organs  men- 
tioned).    To  accomplish  some  of  the  activities,  in  the  higher 
animals  especially,  several  organs  of  a  similar  kind  must  work 
together.     These  are  sometimes  spoken  of  collectively  as  sys- 
tems  of  organs,  e.  g.,  digestive  system,  circulatory  system, 
and  the  like. 

88.  Classification  of  the  Systems  of  Organs  and  Func- 
tions.— The  work  that  needs  to  be  done  by  an  organism  may 
be  considered  under  the  following  heads  :   ( I )  metabolism — 
including  digestion,  circulation,  assimilation,  respiration,  and 
excretion;  (2)  protection  and  physical  support;  (3)  growth; 
(4)  reproduction;  (5)  movement;  (6)  sensation.     Eight  sys- 
tems of  organs  can  be  distinguished  by  which  this  work  is 
done.      They   are    (i)    the   digestive;    (2)    circulatory;    (3) 
respiratory;  (.4)  excretory;   (5)  skeletal  and  integumentary; 
(6)  reproductive;  (7)  muscular,  and  (8)  nervous. 

89.  Metabolism  (Nutrition). — Metabolism  embraces  two 
sets   of  processes,    (i)    constructive   or   anabolic,   known   as 
assimilation,  and  (2)  destructive  or  katabatic.     By  construc- 
tive we  mean  all  the  processes  in  the  organism  which  result  in 
the  storing  of  food  and  energy,  in  growth,  repair,  and  repro- 
duction.    We  class  as  destructive  all  those  processes  by  which 
the  complex  cell  substances  are  broken  down  and  fenergy  set 
free,  leading  to  change  of  temperature,  to  nervous  or  muscular 
action,  to  secretion  and  excretion.     In  the  higher  animals  the 
nutritive  process  is  a  very  complicated  one  and  demands  the 


THE    GENERAL    ANIMAL    FUNCTIONS.  6l 

cooperation  of  numerous  organs.  It  embraces  the  ingestion 
or  taking  in  of  food;  the  digestion  of  food;  its  absorption 
from  the  digestive  tract  into  the  body  fluids — blood  and  lymph ; 
and  its  transportation  in  these  systems,  which  is  made  neces- 
sary by  the  fact  that  digestion  is  confined  to  a  special  region. 
It  likewise  includes  the  further  absorption  of  these  materials 
from  the  blood  and  lymph  by  the  cells  for  whose  benefit  all 
the  preceding  work  has  been  done;  the  assimilative  process 
within  the  cell  whereby  the  food  material  is  made  into  proto- 
plasm or  other  complex  cell-products ;  the  reception  and  trans- 
mission of  oxygen,  by  the  combining  power  of  which  (oxida- 
tion; see  §  25)  these  complex  substances  are  broken  down 
into  simpler  ones — useful,  useless,, or  hurtful  to  the  animal 
economy.  Finally,  the  elimination  of  the  products  of  this 
oxidation  or  burning  is  a  necessary  part  of  the  nutritive 
process.  If  the  material  eliminated  from  the  cell  is  of  further 
use  the  process  is  known  as  secretion,  if  not,  excretion.  It  is 
undesirable  to  attempt  to  make  a  sharp  distinction  between 
excretion  and  secretion. 

90.  The  Digestive  System. — The  simplest  condition  of 
the  digestive  tract  is  found  in  the  gastrula  (arehenteron,  Fig. 
n,  4)  or  in  Hydra  (Fig.  79).  Here  there  is  only  one  cavity 
in  the  body  and  the  food  is  taken  up  immediately  by  the  cells 
needing  it.  A  simple  modification  of  this  condition  is  seen 
in  Fig.  29.  A  still  more  complicated  condition  is  shown  in 
Fig-  93-  I*1  tms  f°rm  which  we  may  take  as  the  type,  the 
digestive  tract  is  a  tube,  running  through  the  body,  lined  with 
its  own  epithelium  and  is  separated  from  the  body  wall  by 
the  cocloni  or  body  cavity.  The  tube  itself  may  have  any 
degree  of  complexity,  but  consists  essentially  of  (i)  an  an- 
terior portion  (stomodceum)  lined  with  ectoderm,  (2)  a  pos- 
terior portion  (proctodoeum)  also  lined  with  ectoderm,  and 
(3)  a  middle  portion  (mesenteron)  lined  with  entoderm.  The 
stomodceum  or  mouth  region  is  usually  supplied  with  devices 
for  the  capture  and  ingestion  of  the  food.  The  mesenteron 
is  the  true  digestive  region.  It  is  supplied  with  cells  which 


62  ZOOLOGY. 

secrete  materials  which  act  upon  foods  in  such  a  way  as  to 
render  them  capable  of  being  absorbed  through  the  entodermic 
cells  into  the  body  cavity,  or  that  special  portion  of  it  known 
as  the  circulatory  system.  Pouches  and  outgrowths  from  the 


c  „ 


FIG.  29.  Stenostoma  (after  Hertwig).  In  this  Turbellarian  the  digestive  tract  (d.t.) 
is  a  blind  sac.  st.,  boundary  of  stomodaeum  and  mesenteron;  c,  cilia;  g,  ganglion 
(brain);  g',  ganglion  of  a  new  individual  which  is  being  formed  by  fission;  o,  mouth;  o', 
mouth  of  new  individual  in  process  of  formation;  w,  excretory  system. 

Questions  on  the  figure. — How  much  of  this  digestive  tract  is  lined 
with  ectoderm?  Which  portion  with  entoderm?  Is  there  a  proctodaeum? 
What  are  the  evidences  that  the  worm  is  in  process  of  division?  Compare 
this  digestive  tract  with  those  in  Figs.  79,  85,  93,  99. 

wall  of  the  mesenteron  are  of  common  occurrence.  These 
serve  to  increase  the  glandular  or  secreting  surface,  the  ab- 
sorbent surface,  and  also  to  retain  the  food  longer  in  contact 
therewith  by  retarding  its  passage  through  the  canal.  The 
removal  of  the  digested  food  from  the  canal  may  be  effected 
by  absorption  or  by  the  active  engulfing  of  food  by  the  ento- 
dermal  cells,  much  as  is  done  by  the  amoeba. 

91.  The  Respiratory  System  and  Function. — In  addition 
to  its  other  food  requirements,  all  protoplasm,  in  proportion 
to  its  activity,  must  have  free  oxygen.  This  is  obtainable  from 
the  air  or  from  the  oxygen  dissolved  in  water.  Oxygen,  being 
a  gas,  must  enter  the  system  in  a  somewhat  different  way  from 
that  by  which  fluids  and  solids  are  ingested.  It  is  best  obtained 


THE    GENERAL   ANIMAL    FUNCTIONS.  63 

by  absorption  through  moist,  thin-walled  membranes.  Such 
surfaces,  in  connection  with  which  blood  vessels  are  usually 
found,  constitute  the  respiratory  organs.  Any  exposed  sur- 
face meeting  these  requirements  may  serve  as  such.  The 
general  surface  of  all  animals  is  respiratory  in  some  degree. 
In  the  more  complex  animals,  however,  special  additional 
organs  must  be  provided.  This  may  be  effected  by  thin  out- 

FIG.  30.  FIG.  31. 


b.  c.  ICl^tecBN  ex. 


FIG.  30.  Diagram  illustrating  gills  or  branchiae,  b.c.,  cavity  in  which  the  body  fluids 
circulate;  br.,  branchial  filaments  which  are  merely  much  thinned  out-pocketings  of  the 
body  wall  (w) ;  ex,  the  external  medium — water — in  which  the  oxygen  is  dissolved. 

Question  on  the  figure. — What  are  the  essential  features  of  gills  as 
suggested  by  this  figure?  Why  are  they  better  suited  to  water  than  to  air? 

FIG.  31.  Diagram  illustrating  lungs  or  tracheae,  b.c.,  the  cavity  in  which  the 
body  fluids  circulate;  /,  the  walls  of  the  lung,  which  are  much  thinned  inpocketings 
of  the  body  wall  (w) ;  ex.,  the  external  medium — usually  the  atmosphere — in  which 
the  oxygen  is  found. 

Questions  on  the  figure. — What  are  the  essential  features  of  lungs  as 
suggested  by  the  figure?  Why  are  such  organs  better  suited  to  aerial  than 
to  aquatic  life?  In  what  respects  are  gills  and  lungs  better  than  the  mere 
body-wall  for  the  exchange  of  the  gases? 

growths  of  the  body  surfaces,  which  are  especially  adapted 
to  water  forms  and  are  called  gills  or  branchice  (Fig.  30)  ;  or  a 
similar  increase  may  be  attained  by  pits  or  ingrowths  of  the 
body  surface,  suited  to  get  oxygen  from  the  air.  Such  are 
called  lungs  (or  trachea)  (Fig.  31).  Carbon  dioxid,  a  gas- 
eous waste  product  resulting  from  the  union  of  oxygen  with 
carbon  which  takes  place  in  the  tissues,  is  economically  elimi- 


64  ZOOLOGY. 

nated  by  the  same  organ  which  admits  the  oxygen,  inasmuch 
as  the  entrance  of  one  gas  is  not  retarded  by  the  outward  pass- 
age of  the  other.  This  double  process  constitutes  respiration, 
although  the  latter  half  is  also  appropriately  described  as  ex- 
cretory. The  surface  devoted  to  the  exchange  of  the  gases- 
and  the  special  devices  necessary  to  renew  the  air  or  water 
make  up  the  respiratory  system.  The  respiratory  organs  are 
frequently  associated  with  the  anterior  or  posterior  end  of  the 
digestive  tract.  As  in  the  case  of  other  foods,  the  blood  is  the 
vehicle  by  which  oxygen  is  distributed  from  the  gills  or  lungs 
to  the  parts  of  the  body  needing  it. 

92.  The  Circulatory  System  and  Function. — In  such  con- 
ditions as  are  shown  in  Fig.  79,  there  is  no  circulatory  system. 
The  digested  food  is  merely  distributed  from  cell  to  cell.  In 
animals  in  which  the  digestive  apparatus  is  well  developed, 
some  device  becomes  necessary  for  the  distribution  of  the 
food.  The  body  cavity  with  its  contained  fluids  may  do  this 
work  as  in  Fig.  29.  Usually  however  when  the  mesodermal 
layers  become  well  developed,  there  arises  therefrom  a  series 
of  branching  tubes  containing  special  fluids,  blood  or  lymph. 
These  tubes  by  their  ramifications  connect  the  digestive  sur- 
faces with  the  various  parts  of  the  body.  Some  branches  like- 
wise extend  to  those  special  surfaces  where  the  oxygen  of  the 
external  medium  may  be  had.  Naturally  then  the  complexity 
and  the  special  structure  of  the  circulatory  system  depend 
largely  upon  the  position  and  degree  of  development  of  the 
digestive  and  respiratory  organs.  In  order  to  secure  the  neces- 
sary motion  of  the  fluids  contained  in  the  tubes,  the  walls  of 
the  latter  are  supplied  with  muscular  fibres,  and  contract  more 
or  less  rhythmically.  If  the  motion  is  to  have  a  definitely  con- 
tinuous direction,  as  is  ordinarily  the  case,  valves  are  usually 
so  placed  that  motion  in  the  opposite  direction  will  be  im- 
possible. The  (one  or  more)  contractile  regions  are  called 
hearts;  vessels  conducting  blood  from  the  heart  are  arteries, 
those  carrying  blood  to\vard  the  heart,  veins.  In  the  region 
where  the  vessels  are  smallest  and  have  very  thin  walls,  the 


THE    GENERAL    ANIMAL    FUNCTIONS.  65 

exchanges  between  the  blood  and  the  other  tissues  occur.  This 
is  the  region  of  capillaries.  The  blood  system  has  capillaries 
in  the  walls  of  the  digestive  tract,  in  the  respiratory  organs, 
and  in  and  about  all  the  tissues  receiving  a  direct  blood  supply. 
The  capillary  region  is  that  for  which  the  rest  exists ;  it  is  the 

FIG.  32. 


FIG.  32.  A  scheme  to  represent  the  circulation  of  the  blood,  in  its  essential  features. 
The  arrows  indicate  the  course  of  the  blood,  a,  arteries;  aur.,  auricle  or  receiving  por- 
tion of  the  heart;  d,  digestive  tract;  c.  d.,  capillaries  of  the  digestive  tract;  c.r.,  capil- 
laries of  the  respiratory  organs;  c.s.,  capillaries  of  the  system;  va.,  valves;  ve,  veins; 
vt.,  ventricle. 

Questions  on  the  figure. — What  portions  of  the  apparatus  are  neces- 
sary to  secure  circulation?  Which  secure  the  real  objects  for  which  the 
circulation  .exists  ?  Why  are  valves  essential  ?  What  common  work  occurs 
in  the  three  classes  of  capillaries  figured  above?  What  special  type  of 
work  is  characteristic  of  each  of  the  three? 

physiologically  important  part  of  the  system.  Fig.  32  illus- 
trates the  arrangement  of  parts  found  in  a  common  type  of 
circulatory  apparatus. 

93.  Demonstration. — Circulation  of  blood   in   tail   of  tadpole;   in  the 
web  of  the  foot  of  a  frog;  or  in  the  fin  of  small  fish.     Distinguish  veins 
and  arteries.     Notice  behavior  of  corpuscles  in  passing  through  small  capil- 
laries.    Compare  rate  of  flow  in  vessels  of  different  size. 

94.  The  Excretory  System  and  Function. — Beside  the 
carbon  dioxid  eliminated  from  the  blood  in  the  lungs  or  gills, 
other  waste  products  of  oxidation  are  to  be  removed  from  the 
tissues  where  they  are  produced.     Important  among  these  are 

6 


66  ZOOLOGY. 

the  nitrogenous  wastes,  urea  and  uric  acid.  In  organisms  in 
which  there  is  no  regular  blood  system,  these  waste  products 
may  be  carried  directly  from  the  tissues  to  the  surface  by  a 
system  of  tubes  beginning  as  capillaries.  In  the  majority  of 
animals  the  canals  (nephridia)  pass  from  the  body  cavity  to 
the  exterior.  These  are  seen  in  a  simple  condition  (Fig.  33) 

FIG.  33. 


FIG.  33.  Diagram  of  a  nephridium  (simple  kidney  tubule)  of  a  Segmented  Worm. 
b.,  b'.,  blood  vessels;  c,  ccelome;  d,  duct  of  the  nephridium;  e,  external  opening;  cf, 
ciliated  funnel  opening  into  ccelome;  gl,  glandular  or  secreting  portion;  s,  septum; 
W,  body  wall  composed  of  longitudinal  muscle  fibres,  circular  fibres,  and  epithelial 
layer;  w,  wall  of  gut. 

Questions  on  the  figure. — Judging  from  its  relation  to  the  coelome,  to 
the  blood  vessels,  and  to  the  outside  world  what  would  seem  a  reasonable 
function  for  the  nephridium? 

in  the  segmented  worms.  For  a  more  complicated  condition 
see  unsegmented  worms  (Figs.  86-88).  The  kidneys  of 
higher  forms  are  considered  to  be  derived  from  these.  In  the 
higher  animals  the  kidneys  have  a  special  blood  supply  and 
the  waste  products  are  extracted  from  the  blood  while  in  the 
kidney. 

95.  The  Skeletal  System  and  its  Functions. — The  cells  of 
the  body  frequently  excrete  from  themselves  materials  which, 
while  no  longer  of  use  to  the  protoplasm,  are  not  entirely  re- 
moved from  the  body  by  the  blood  and  serve  important  passive 
functions.  These  excretions  or  secretions  may  surround  and 
protect  and  give  rigidity  to  the  cell  itself  (e.  g.,  cell- walls; 
shells  in  the  single-celled  animals),  may  bind  the  cells  together 


THE    GENERAL    ANIMAL    FUNCTIONS.  67 

into  a  resistant  tissue  (intercellular  substance  in  bone,  etc.), 
or  may  be  secreted  at  the  surface  of  the  organism  as  a  whole 
(cuticula  in  insects  and  shell  in  mollusks).  The  hard  parts 
serve  primarily  for  the  support  and  protection  of  the  softer 
tissues.  Incidentally  they  come  to  serve  a  very  important  use 
as  points  of  attachment  for  muscles.  The  skeleton  may  be 
external  (the  integumentary  skeleton,  or  exoskeleton)  as  in 
crayfish,  or  internal,  as  the  endoskeleton  of  vertebrates.  In 
many  instances  both  kinds  of  skeletal  structures  may  occur 
simultaneously,  yet  it  is  usually  true  that  if  the  exoskeleton 
is  well  developed  the  endoskeleton  will  be  poorly  represented. 
Each  has  important  advantages  and  limitations.  To  allow 
motion  as  the  result  of  muscular  action  the  skeleton,  if  rigid, 
evidently  must  be  in  segments  and  jointed. 

96.  Growth. — There  are  no  special  organs  of  growth,  yet 
growth  is  one  of  the  most  immediate  and  important  manifesta- 
tions of  the  nutritive  process.     Growth  is  to  be  defined  as  in- 
crease in  volume  or  mass  and  may  result  from  either  or  all  of 
three  processes:  viz.  (i)  absorption  of  water,  (2)  formation 
of  protoplasm  and  the  multiplication  of  cells,  and  (3)  forma- 
tion of  non-protoplasmic  cell-products,  either  within  or  among 
the  cells. 

The  rate  and  character  of  growth  are  modified  by  such 
external  conditions  as  temperature,  light,  quantity  and  quality 
of  the  food  supply,  etc.  Growth  does  not  continue  indefinitely. 
Its  continuance  is  determined  by  the  relation  of  the  anabolic 
to  the  katabolic  processes  in  the  body.  The  time  comes  in  the 
life  of  every  complex  organism  when  the  income  no  longer 
equals  the  outgo,  and  growth  must  cease.  Later  still  the  wear 
is  not  made  good  by  the  income,  and  death  results. 

97.  Reproduction  and  the  Reproductive  Organs. — Since 
individual  organisms  are  limited  both  with  regard  to  growth 
and  length  of  life,  it  is  apparent  that  a  given  class  of  forms 
cannot  continue,  unless  some  method  of  originating  new  in- 
dividuals be  found.     This  production  of  new  individuals  by 


68  ZOOLOGY. 

the  instrumentality  of  the  old  is  reproduction.  In  many  of 
the  lower  animals  this  is  merely  a  growth  process, — "  growth 
beyond  the  limits  of  the  individual."  In  the  single-celled  ani- 
mals reproduction  means  the  formation  of  the  protoplasm  into 
two  or  more  masses,  by  dividing  into  two  equal  parts  (divis- 
ion), by  breaking  into  a  large  number  of  sub-equal  portions 
(fragmentation),  or  by  budding  (Chapter  III,  §  39).  In  bud- 
ding there  is  the  formation  of  a  local  outgrowth  which  ulti- 
mately attains  the  size  and  character  of  the  parent.  In  division 
the  resulting  individuals 'cannot  be  distinguished  as  parent  and 
offspring.  Such  reproduction,  involving  only  one  parent,  is 
asexual.  It  usually  occurs  when  the  adult  size  of  the  animal 
is  attained.  It  is  not  confined  to  the  Protozoa  or  single-celled 
animals,  but  may  occur  in  several  Invertebrate  groups  in  which 
(Hydra,  Fig.  79)  there  is  not  a  high  degree  of  specialization. 
The  budded  individual  or  offspring  may  in  such  cases  consist 
of  one  cell  or  of  many.  In  addition  to  the  stimulus  afforded 
by  the  attainment  of  normal  size,  external  conditions  such  as 
diminished  food  supply,  temperature  changes,  etc.,  influence 
the  process  of  non-sexual  reproduction. 

98.  Sexual  Reproduction. — It  seems  for  some  reason  that 
even  in  the  one-celled  animals  the  method  of  reproduction  by 
division  cannot  be  continued  indefinitely  without  ill  effects  to 
the  organism.  In  many  Protozoa  there  is  at  certain  times  a 
union  of  two  individuals,  either  temporarily  or  permanently, 
accompanied  by  exchange  of  nuclear  material  or  a  fusion  of 
the  whole  protoplasm.  After  a  period  of  rest  division  begins 
again  with  renewed  activity.  Something  similar  is  seen  in 
the  more  complex  animals — the  Metazoa.  After  a  period  of 
cell  divisions,  by  which  the  individual  body  is  built  up,  the 
majority  of  cells,  as  muscle  or  nerve  cells,  appear  to  lose  their 
power  of  dividing,  and  even  the.  less  differentiated  cells  which 
we  have  described  as  the  ova  and  sperm  are  incapable  of  con- 
tinuing the  division  necessary  to  produce  a  new  individual  until 
they  have  been  stimulated  by  union  with  each  other  (or  by 
seme  artificial  means).  Such  unions  of  cells,  to  form  by  later 


THE    GENERAL    ANIMAL    FUNCTIONS.  69 

divisions  a  new  individual,  are  called  conjugation  or  fertilisa- 
tion, and  the  new  individual  which  results  is  said  to  arise  by 
sexual  reproduction.  The  uniting  cells  may  be  similar  (as  in 
Pandorina),  in  which  case  the  union  is  isogamous.  More 
usually  the  cells  are  different  and  the  union  is  heterogatnous. 
In  the  latter  case  the  cells  are  called  ovum  and  sperm  (Chapter 
IV)  and  are  usually  formed  by  different  individuals,  though 
very  often  the  same  individual  may  produce  both  classes  of  cells 
(hermaphroditism}  from  different  regions  of  the  germinative 
epithelium,  or  in  the  same  organ  at  different  times.  The 
special  organs  in  which  the  ova  are  produced  are  called  ovaries. 
The  sperm  cells  are  formed  in  testes.  The  individuals  (that 
is,  the  male  and  female)  producing  the  different  classes  of 
cells  are  often  very  different  in  other  respects.  This  is  known 
as  sexual  dimorphism  (Chapter  VII,  §  145). 

99.  Practical  Exercises. — Compare  the  males  and  females  of  the  vari- 
ous animal  types  with  which  you  are  familiar.     In  what  groups  of  animals 
does  non-sexual  reproduction  occur?     Give  the  gist  of  Geddes  and  Thomp- 
son's theory  as  to  the  origin  of  sexuality.     Compare  any  other  theories 
available  to  you.     What  are  the  conceivable  advantages  and  disadvantages 
of  the  asexual  method?  of  the   sexual?  of  hermaphroditism?   of  sexual 
dimorphism? 

100.  Movement  and  the  Muscular  System. — The  desir- 
ability of  motion  in  animals  arises  from  the  necessity  of  seek- 
ing food  and  of  escaping  unfavorable  influences.    These  con- 
ditions constitute  the  most  imperious  stimuli  to  which  the  or- 
ganism is  subject.     We  have  already  seen  (§§  19,  23)  that  the 
fundamental  irritability  and  contractility  of  protoplasm  make 
this  response  possible  in  the  simplest  conditions.    In  somewhat 
higher  forms,  specially  developed  protoplasmic  fibrils  appear, 
such  as  cilia,  or  the  fibrils  in  the  stalk  of  Vorticella,  in  which 
the  power  of  contracting  is  strikingly  manifest  (see  Figs.  66 
and  68).     While  this  is  found  in  Protozoa,  it  is  much  mo^e 
clearly  shown  in  the  muscular  tissue  (Fig.  28)  of  stil)  higher 
animals.    Locomotion  varies  in  efficiency  in  different  animals 
not  merely  on  account  of  varying  muscular  structure  but  in 
accordance  with  the  arrangement  of  the  hard  parts  to  which 


7O  ZOOLOGY. 

the  muscle  fibres  are  attached,  and  the  nature  of  the  medium 
which  must  be  penetrated.  Many  aquatic  forms,  though  free- 
swimming  in  their  early  stages,  may  become  attached  and  give 
up  the  power  of  locomotion  in  the  adult  condition.  Such 
attached  forms  ordinarily  secrete  an  external  shell  or  covering 
into  which  they  can  withdraw  for  protection  (e.  g.,  barnacles, 
many  polyps ) .  They  must  depend  upon  currents  in  the  water 
to  supply  them  with  food.  They  are  frequently  able  to  produce 
the  currents  by  the  motion  of  parts  of  the  body.  The  majority 
of  active  movers  have  hard  parts  which  serve  as  levers  to 
which  the  muscles  are  attached.  The  parts  of  the  skeleton, 
which  may  be  either  external  to  the  muscles  or  surrounded  by 
them,  articulate  with  one  another  by  a  hinge  or  movable  joint, 
as  illustrated  by  vertebrates  or  insects.  In  some  forms  with- 
out a  conspicuous  skeleton,  as  the  earthworm,  there  is  a 
dermo-muscular  wall  surrounding  a  fluid-filled  cavity.  By  the 
alternate  use  of  the  longitudinal  and  circular  fibres,  changing 
the  relative  position  of  the  parts  of  the  body,  locomotion  is 
effected.  The  special  appendages,  particularly  the  paired 
appendages,  are  important  motor  organs  in  nearly  all  actively 
moving  animals. 

101.  Sensation  and  Sensory  Structures. — In  a  simple  bit 
of  protoplasm  it  is  manifest  that  the  differences  between  the 
living  matter  and  the  outside  world  are  greater  than  the  struc- 
tural differences  between  the  parts  of  the  protoplasm  itself. 
Thus  we  would  expect  the  stimuli  arising  from  the  environ- 
ment to  be  among  the  most  important  experienced  by  the 
organism,  and  that  the  superficial  protoplasm  by  virtue  of  its 
irritability    (see  also  §    19)    would  most  promptly  feel  and 
respond  to  such  stimuli.     The  changes  thus  instituted  will  be 
felt  sooner  or  later  to  the   remotest  parts  of  the  cell  mass. 
This  transfer  of  the  effects  of  a  stimulus  through  a  longer  or 
shorter  distance  introduces  us  to  a  second  nervous  function, 
— internal  irritability  of  protoplasm  or  conductivity. 

102.  As  an  organism  increases  in  the  number  and  variety 
of  its  cells,  the  specialized  structures  need  to  be  more  com- 


THE    GENERAL    ANIMAL    FUNCTIONS.  7 1 

pletely  bound  to  one  another.  It  becomes  necessary  not  only 
that  they  receive  impulses  from  such  parts  as  are  favorably 
situated  for  the  reception  of  stimuli,  but  that  a  degree  of 
coordination  of  the  interrelated  parts  be  secured,  in  order  that 
just  such  response  shall  be  made  as  will  best  meet  the  needs 
of  the  organism.  This  power  of  coordinated  response  to  ex- 
ternal stimuli  makes  it  possible  for  an  organism  to  become 
suited  to  its  environment. 

103.  In  the  higher  forms,  the  work  above  described  de- 
mands five  classes  of  structures  (see  Fig.  34);  (i)  end  or 
sense  organs,  which  are  specially  sensitive  to  stimuli  of  dif- 
ferent orders,  as  mechanical  (touch),  chemical,  ethereal 
(light),  etc.;  (2)  conductive  tracts  (afferent  nerves),  which 

FIG.  34- 


FIG.  34.  Scheme  showing  the  essential  relations  of  the  parts  of  a  nervous  system:  i, 
the  sensory  end  organ  (epithelial);  2,  afferent  nerve  tract;  3,  central  nervous  cells 
(ganglia);  4,  efferent  nerves,  leading  to  5,  muscle,  gland,  etc.  g,  ganglion  cells;  gl., 
gland;  m,  muscle  fibre;  n.f.,  nerve  fibre;  s.e.,  sensory  epithelium. 

Questions    on   the   figure. — What   seems   to   be   the   function   of  the 
various  parts  or  elements  in  this  scheme?    Your  reasons  for  your  view? 

connect  (i)  with  (3)  central  nervous  structures  (ganglion- 
cells)  where  the  impulse  is  received  and  suitable  responsive 
impulses  are  originated;  (4)  conductive  tracts  (efferent 
nerves),  which  make  the  work  of  the  central  organs  of  value 
by  carrying  an  impulse  which  produces  corresponding  activi- 


ZOOLOGY. 


ties  in  (5)  some  form  of  related  cells:  muscular,  glandular, 
or  nervous.  It  is  readily  apparent  how  increase  of  volume  and 
differentiation  of  the  other  parts  will  make  necessary  a  more 
complicated  nervous  system.  The  special  arrangement  of  these 
parts  of  a  complete  system  differs  very  much  in  various  animal 
groups,  yet  it  may  be  said  that  there  is  a  progressive  accumula- 
tion of  the  central  nervous  matter  at  the  anterior  end  of  the 
body  as  we  ascend  the  scale  of  animal  life.  When  this  con- 
centration is  well  advanced  the  mass  of  nervous  matter  is 
called  the  central  nervous  system  which  always  includes  the 
brain.  The  nerves  passing  to  and  from  the  central  part  and 
their  endings,  taken  collectively,  are  described  as  the  peripheral 
nervous  system. 

104.  Arrangement  of  the  Central  Nervous  System. — The 
ganglion  cells  composing  the  nervous  system  may  be  so  scat- 
tered through  the  superficial  layers  as  scarcely  to  deserve  the 
name  "  central  "  (Hydra).  The  nerve  cells  may  be  arranged 

FIG.  35. 


FIG.  35.     Diagram  showing  arrangement  of  the  nervous  matter  in   Starfish,     c,  gangli- 

onated  ring  about  the  mouth;  o,  mouth;  r.n.,  radial  nerve  in  each  arm. 

FIG.  36.     The    nervous    system    of    the    Clam, — from    the    dorsal    aspect,     a,    anterior; 

o,  mouth;  e.g.,  cerebral  ganglia  (brain);  p.g,  pedal  ganglia;  v.g.,  visceral  ganglia. 

Questions  on  Fig.  35. — Describe  in  your  own  terms  the  way  in 
which  the  principal  nerve  elements  are  arranged  in  the  starfish.  Compare 
it  with  those  which  follow.  In  what  respects  similar?  In  what  unlike 
them? 


THE    GENERAL    ANIMAL    FUNCTIONS. 


73 


in  a  ring  about  the  mouth,  or  gullet,  with  or  without  addi- 
tional bands  of  nervous  tissue  containing  cells  passing  radially 
from  it  (as  in  echinoderms  and  some  coelenterates ;  see  Fig. 
35).  In  the  higher  invertebrates  this  process  of  concentration 
continues  and  the  ganglionic  cells  are  collected  into  two  or 
more  ganglia  connected  by  nerve  fibres  (commissures).  Usu- 

FIG.  37- 


FIG.  37.  Arrangement  of  the  nervous  material  in  the  anterior  end  of  an  Oligochete 
Worm,  seen  in  profile.  That  part  of  the  body  wall  nearest  the  observer  is  supposed  to 
be  removed,  a,  anterior;  b.w.,  body  wall;  g,  dorsal  ganglia  (brain);  g',  ventral  chain 
of  ganglia;  n,  nerve  ring  around  the  pharynx;  o,  mouth;  p,  pharynx. 

FIG.  38.     The  central  nervous  system  in  a  Leech.     Lettering  as  in  Fig.  37. 

Questions  on  figures  36,  37,  and  38. — How  is  the  nervous  matter  re- 
lated to  the  digestive  tract  and  to  the  animal  as  a  whole  in  all  of  these 
figures?  Compare  with  figures  in  other  texts.  Is  there  any  apparent 
correlation  between  the  form,  symmetry  or  segmentation  of  the  animal 
and  the  arrangement  of  the  nervous  material?  Can  you  state  your  con- 
clusion as  a  law?  t.J... 

ally  a  pair  of  ganglia  occurs  in  the  region  of  the  mouth,  and 
dorsal  thereto  (e.  g.,  clam,  Fig.  36).  In  segmented  forms, 
as  the  earthworm  and  crayfish,  there  is  also  a  series  of  gang- 
lia connected  by  fibres,  ventral  to  the  digestive  tract.  This 
chain  is  in  turn  connected  with  the  dorsal  ganglia  by  a  loop 
of  fibres  passing  round  the  oesophagus  (Figs.  37  and  38). 


74  ZOOLOGY. 

In  vertebrates  the  central  nervous  system  consists  primarily 
of  a  tube  with  nervous  walls — the  spinal  cord — which  may  be 
specially  enlarged  and  thickened  at  the  anterior  end  to  produce 
the  brain  (Fig.  170).  From  the  various  parts  of  this  cord  the 
nerves  take  their  origin. 

105.  The  Peripheral  Nervous  System:  Sense  Organs. — 

We  know  by  experimentation  that  in  the  lowest  animals  even, 
or  for  that  matter,  in  protoplasm,  certain  external  conditions 
produce  definite  responses  or  changes.  We  also  know  that 
these  external  happenings  and  their  responses,  in  our  own 
case,  are  accompanied  by  certain  sensations,  as  touch  or  taste. 
By  inference,  both  from  the  nature  of  the  response  and  from 
the  structure  of  organs,  we  reach  the  conclusion  that  the  lower 
vertebrates  and  higher  invertebrates  experience  sensations  in 
some  degree  similar  to  our  own.  The  classes  of  possible 
stimuli  have  already  been  mentioned  (§  20).  Those  producing 
in  us  definite  sensations  are :  simple  contact  stimuli,  producing 
the  sensation  of  touch  and  pressure;  vibratory  contacts,  giving 
rise  to  hearing  and  temperature  sensations;  chemical  actions, 
making  possible  sensations  of  taste  and  smell ;  ethereal  vibra- 
tions, producing  the  sensation  of  light.  In  the  lowest  forms 
of  animals  there  are  no  specialized  organs  for  the  reception  of 
particular  stimuli,  and  in  such  cases  it  is  reasonable  to  infer 
that  the  distinctness  of  the  sensation  cannot  be  very  great.  In 
almost  all  animals,  however,  certain  areas  are  specially  suited 
to  be  stimulated  by  special  stimuli. 

106.  Touch. — There  are  two  principal  ways  by  which  contact  stimuli 
are  received  among  animals.     Fibres  of  the  central  nervous  system  may 
pass  to  the  skin  and  end  among  its  outer  layers  as  free  nerve  terminations, 
or  these  fibres  may  become  intimately  united  with  one  or  more  of  the 
cells  of  the  epithelium.     The  most  common  of  the  tactile  organs  in  verte- 
brates are  of  the  first  class.     Where  the  stimulus  reaches  the  nerve  through 
a  nervous  epithelium,  the  epithelial  cells  often  have  special  developments 
such  as  hairs,  bristles,  and  the  like,  whereby  the  possibility  of  contact  with 
external  objects  is  increased.     The  appreciation  of  changes  in  temperature 
is  also  associated  with  the  general  skin  surface. 

107.  Chemical    Sense    (including   taste   and   smell). — It   is    impossible 
for  us  to  distinguish  between  taste  and  smell  in  the  lower  animals.     Indeed 


THE    GENERAL    ANIMAL    FUNCTIONS.  75 

it  is  with  difficulty  that  we  separate  the  sensations  obtained  from  the  two 
sources  even  in  our  own  case,  in  some  instances.  Almost  all  animals  seem 
to  have  some  power  of  appreciating  the  chemical  condition  of  the  medium 
in  which  they  live.  In  aquatic  animals  the  chemical  sense  organs  may  be 
distributed  over  the  surface  of  the  body.  In  the  higher  animals  they  col- 
lect more  and  more  at  the  anterior  or  mouth-end  of  the  animal,  with 
manifest  advantage  to  the  animal.  In  the  higher  land  forms,  especially 
the  vertebrates,  the  organs  of  the  chemical  sense  come  to  lie  in  or  about 
the  mouth  and  nose,  —  the  beginnings  of  the  digestive  and  respiratory  tracts 
respectively.  These  senses  are  specially  related  to  the  testing  of  food  and 
the  medium  in  which  the  animal  lives.  The  senses  thus  far  enumerated 
seem  among  the  earliest  developed  in  the  animal  kingdom. 

108.  Hearing.  —  It  is  by  no  means  certain  that  the  lower  animals  pos- 
sess the  ability  to  appreciate  the  vibrations  in  matter  (air,  water,  etc.), 
which  arouse  in  us  the  sensation  of  sound.  There  are  in  several  groups 
of  such  animals  organs,  the  structure  of  which  would  suggest  that  they 
might  receive  vibrations  of  the  medium  in  which  they  live.  In  their  sim- 
plest condition  they  consist  of  a  sac  (otocyst}  derived  from  the  ectoderm 
and  lined  by  an  epithelium  containing  sensory  cells.  From  these  cells 

FIG.  39. 


s.c. 


FIG.  39.     Otocyst   in    a   Mollusk.     n,    nerve;    o,   otolith;    s.c.,    sensory   cells  in   wall   of 
otocyst.     (After  Claus.) 

Questions  on  the  figure. — What  immediately  stimulates  the  sensory 
epithelium  in  this  case?  What  kinds  of  general  agencies  might  be  sup- 
posed to  produce  the  necessary  motion  for  this  purpose?  What  is  the 
present  view  of  the  function  of  otocysts? 

sensory  hairs  extend  into  the  cavity  (Fig.  39).  The  cavity  contains  a 
fluid  which  may  support  one  or  more  solid  particles  (otoliths).  With  the 
vibration  of  the  medium  the  whole  would  be  put  into  vibration,  but  the 
inertia  of  the  contained  fluid  and  otoliths  would  cause  the  latter  to  strike 
against  the  hairs  and  thus  serve  as  stimuli  to  the  sensory  cells.  Late 
researches  tend  to  prove  that  these  structures  are  organs  for  preserving 
equilibrium  rather  than  of  hearing.  In  higher  forms  the  ear  becomes  im- 
mensely more  complex,  but  the  general  conditions  both  of  origin  and 


76 


ZOOLOGY. 


structure  appear  to  be  much  as  described  for  the  otocysts.  In  some  of  the 
lower  animals,  as  insects,  there  are  also  external  vibratile  hairs  which  are 
believed  to  be  auditory  (Fig.  40). 

FIG.  40. 


FIG.  40.     Antenna  of  Male  Mosquito   (.Culex  pipiens).     By  J.  W.   Folsom. 

Questions  on  the  figure. — Compare  with  the  antennae  of  a  female  (see 
Fig.  63).  What  are  the  differences  between  the  head  of  the  male  and 
female  mosquitoes?  What  is  believed  to  be  the  function  of  these  plumose 
antennas?  What  are  the  evidences  for  this  view? 

109.  Sight. — There  are  three  distinct  facts  to  be  noted  with  respect  to 
visual  sensation  in  the  higher  forms  of  animals:  the  perception  of  light, 
the  perception  of  color  (».  e.,  light  of  different  wave-frequency)  and  the 
formation  of  images  of  external  objects.  It  has  already  been  seen  (§20) 
that  protoplasm  is  sensitive  and  responsive  to  light  without  any  special 
organs.  The  simplest  visual  organs  found  in  multicellular  animals  con- 
sist merely  of  epithelial  cells  containing  pigment  in  which  changes  are 
wrought  apparently  by  the  action  of  light  (Fig.  41,  a).  These  changes 
affect  the  nerve  fibres  associated  with  the  pigment  cells  and  thus  the 
central  nervous  organ.  Such  eyes  are  capable  only  of  giving  knowledge 
of  the  intensity  or,  if  properly  constructed,  of  intensity  and  direction  of 
the  light  and  do  not  form  an  image  of  external  objects.  There  are 
several  types  of  image-forming  eyes  in  the  animal  kingdom.  The  most 
familiar  of  these  is  the  "camera  eye"  of  vertebrates,  so  called  because 


THE    GENERAL    ANIMAL    FUNCTIONS. 


77 


FIG.  41.  Diagrams  showing  some  of  the  stages  in  the  increasing  complexity  of  the 
simple  eye  in  Invertebrates.  A,  simple  pigment  spot  in  epithelium  having  nerve-endings 
associated  with  pigment  cells  (as  in  some  medusae) ;  B,  pigment  cells  in  a  pit-like  de- 
pression (as  in  Patella') ;  C,  with  pin-hole  opening  and  vitreous  humor  in  cavity  (as 
in  Trochus) ;  D,  completely  closed  pit,  with  lens  and  cornea  (as  in  Triton  and  many 
other  Mollusks) ;  E,  pigment  area  elevated  instead  of  depressed,  lens  of  thickened 
cuticula  (as  in  the  Medusa,  Lizsia) ;  F,  retinal  cells  more  highly  magnified,  ep., 
epidermis;  f,  nerve  fibre;  /,  lens;  op,  optic  nerve;  p,  pigment  cells;  r,  retina;  v.h,, 
vitreous  humor. 

Questions  on  the  figures. — What  changes  take  place  in  the  sensory 
epithelium  in  this  series  of  figures?  What  is  gained  by  such  a  depression 
as  occurs  in  B?  What  purpose  is  served  by  the  pinhole  and  the  vitreous 
humor  of  C?  Describe  the  change  from  C  to  D.  What  is  gained?  What 
may  be  the  function  of  the  pigment?  Compare  texts.  In  what  respects 
does  E  differ  from  the  other  types?  What  two  types  of  cells  are  figured 
as  belonging  to  each  retina?  What  constitutes  the  retina? 

it  illustrates  the  principles  made  use  of  in  the  construction  of  the  photo- 
graphic camera.  In  this  there  is  a  lens  or  body  which  refracts  the  rays 
of  light  in  such  a  way  that  all  the  rays  passing  from  a  point  in  the  object 
are  brought  to  a  focus  at  a  point  on  the  retina.  Another  type  of  image- 
forming  eye  is  the  compound  eye  of  insects  and  Crustacea  (Fig.  42). 
These  are  made  up  of  a  large  number  of  eye  elements — each  structurally 
complete  in  itself — whose  separately  formed  images  must  nevertheless  be 
joined  in  order  to  form  a  picture. 

The  degree  to  which  the  color-sense  is  developed  among  lower  animals 
is  very  uncertain.  The  simplest  animals  may  respond  differently  to  light 
of  different  colors,  but  this  is  a  very  different  thing  from  saying  that  they 
possess  the  color-sense. 


78  ZOOLOGY. 

To  summarize, — the  essential  part  of  the  eye  is  the  sensitive  layer 
known  as  the  retina.  The  other  parts  of  the  complex  eye-structure  serve 
the  purposes  of  shutting  out  the  light  except  from  certain  directions;  of 
focusing  the  light  admitted  in  such  a. way  as  to  increase  its  intensity  and 
form  an  image  on  the  retina;  of  adjusting  this  apparatus  to  objects  at 

FIG.  42. 


•—£/>. 


FIG.  .42  Diagram  illustrating  the  compound  eye  of  arthropods.  A,  the  whole  eye 
shown  in  section;  B,  one  of  the  eye-elements  (ommatidium)  more  highly  magnified,  c, 
cuticular  facets;  ep,  epidermis;  /,  group  of  cells  forming  lens-like  body;  n,  optic  nerve 
fibres;  o,  optic  ganglia;  p,  pigment  cells. 

Questions  on  the  figure. — In  what  way  is  the  independence  of  each 
ommatidium  secured?  In  other  words  what  is  to  prevent  the  light  which 
comes  in  obliquely  from  passing  from  one  ommatidium  to  another?  In 
what  conceivable  way  is  a  general  image  obtained  from  these  various 
partial  views  ?  What  groups  of  animals  possess  eyes  of  this  sort  ?  Compare 
the  diagram  B  with  the  figure  oftthe  complete  ommatidium  of  the  lobster 
(Fig.  125). 

different  distances ;  of  nourishing  and  supporting  the  more  important 
portions  of  the  apparatus;  and  of  moving  the  eye  so  as  to  take  into  view 
different  portions  of  the  surroundings.  Some  of  the  various  grades  of 
compexity  of  eye-structure  in  the  Invertebrate  series  beginning  with  a 
pigment  spot  and  ending  with  a  complete  lens-eye,  are  shown  in  Fig.  41. 

no.  Analogy  and  Homology. — In  comparing  various 
animals  we  find  that  they  may  do  the  same  work  with  organs 
that  arise  in  very  different  ways,  which  however,  because  they 
are  adapted  to  perform  similar  tasks,  look  somewhat  alike. 


THE    GENERAL    ANIMAL    FUNCTIONS. 


79 


Such  structures  are  said  to  be  analogous  (as  the  wing  of  a 
bird  and  the  wing  of  a  butterfly).  In  other  cases  organs 
that  originate  in  the  same  way  may  have  been  used  so  differ- 
ently as  to  have  a  very  different  appearance,  as  the  various 

FIG.  43. 


hres. 


FIG.  43.  Diagrams  illustrating  two  stages  in  the  development  of  the  vertebrate  eye. 
A,  showing  the  relation  of  the  ectoderm,  the  brain  vesicle,  and  the  optic  vesicle.  The 
right  side  of  the  figure  shows  a  later  stage  than  the  left.  B,  later  stage,  showing 
the  lens,  eye-ball  and  retina  in  position,  b.v.,  brain  vesicle  formed  by  the  invagina- 
tion  of  the  ectoderm  (ect.~) ;  /,  lens;  mes.,  mesodermal  tissue;  o.n.,  optic  nerve;  o.s., 
optic  stalk;  o.v.,  optic  vesicle,  a  portion  of  the  brain  vesicle;  r,  retinal  layer;  v.h., 
interior  of  eye-ball  which  comes  to  contain  the  vitreous  humor. 

Questions  on  the  figures. — Which  portions  of  the  eye  are  derived 
directly  from  the  ectoderm?  Which  indirectly,  *'.  e.,  from  the  brain? 
Which  portions  seem  of  mesodermal  origin?  By  following  the  invagination 
by  which  the  retina  is  formed  do  you  find  any  suggestion  of  an  explanation 
of  the  fact  that  the  sensitive  portion  of  the  retina  (rods  and  cones,  Fig. 
173)  is  directed  away  from  the  light?  Refer  to  some  work  on  the  embry- 
ology of  the  vertebrates  for  more  complete  series  of  figures. 

"  legs  "  of  the  crayfish,  or  the  wing  of  a  bird  and  the  arm 
of  man.  These,  notwithstanding  their  superficial  differences, 
are  said  to  be  homologous  because  of  the  fundamental  equival- 
ency of  structure. 

in.  Summary. 

i.  Division  of  labor  and  differentiation  of  structure  proceed 
together  as  the  individual  develops.  All  tissues  retain  the 
power  of  using  food,  of  oxidation,  of  eliminating  useless  prod- 
ucts. Other  functions  incidental  to  these  may  be  relegated  to 
special  cells  or  tissues. 


So  ZOOLOGY. 

2.  Associations  of  tissues  to  accomplish  a  more  or  less  defi- 
nite work  are  called  organs :  organs  of  a  similar  kind  are  col- 
lectively known  as  systems  of  organs. 

3.  The  principal  functions  of  animals  and  the  organs  or 
systems  performing  the  work  may  be  classed  as  follows : 

Function.  System. 

(a)  Metabolism Nutritive. 

(6)  Support  and  protection ..  Skeletal,  and  integumentary. 

(c)  Growth 

(d)  Reproduction  .  . Reproductive. 

(e)  Motion   Muscular,   in   connection   with 

skeletal. 
(/)   Sensation   Nervous. 

4.  Metabolism  or  nutrition  embraces  the  following  proc- 
esses : — 

(a)  Ingestion  of  food  (including  oxygen), 

(b)  Digestion, 

(c)  Absorption, 

(d)  Circulation, 

(e)  Assimilation  =  anabolism, 
(/)   Dissimilation  =  katabolism, 

(g)   Secretion  and  excretion  (of  waste  matter  including  car- 
bon dioxid). 

The  processes  in  (a),  (b),  (c),  and  (e)  are  anabolic,  i.  e., 
add  to  the  resources  of  the  body.  Those  in  (/)  and  (g)  are 
katabolic,  i.  e.,  tend  to  destroy  the  materials,  develop  energy, 
and  eliminate  waste.  Circulation  contributes  to  the  accom- 
plishment of  both  purposes. 

5.  The  supportive  skeletal  structures  may  be  internal,  or 
external,  or  both.    They  may  arise  as  a  secretion  of  the  super- 
ficial cells  of  the  body,  or  consist  of  a  mixture  of  cells  and 
intercellular  substance.     Their  nature  and  arrangement  are 
profoundly  important  in  determining  the  distribution  of  the 
other  more  active  organs. 

6.  Growth  and  reproduction  are  the  outcome  of  the  nutri- 


THE.  GENERAL   ANIMAL    FUNCTIONS.  8 1 

tive  processes.  Growth  is  increase  in  mass;  reproduction  is 
the  production  of  new  individuals  from  old.  Reproduction 
always  involves  cell  division  and  may  be  asexual  or  sexual. 
The  latter  normally  involves  two  parents.  In  it  two  cells, 
which  may  be  either  similar  or  dissimilar,  must  unite  before 
development  will  proceed. 

7.  The  nervous  and  muscular  systems  are  closely  related 
in  function.     Their  united  work  is  to  receive,  coordinate  and 
respond  to  the  external  or  internal  stimuli  affecting  the  ani- 
mal.     The   structures   to   receive   stimuli    (end   organs)    are 
largely   superficial;    the   coordinating   and   controlling   parts 
(central  organ)  are  deep-seated,  thereby  securing  protection 
and  a  central  position ;  the  muscular  system  must  have  definite 
relations  with  the  hard  parts  upon  which  it  acts.  Thus  arises 
the  necessity  of  connectives  or  nerves  between  the  various 
portions. 

8.  The  sense  organs  represent  areas  of  the  epithelium  which 
are  peculiarly  adapted  to  the  reception  of  some  one  of  the 
forms  of  stimulus  to  which  animals  are  subject,  supplemented 
by  a  more  or  less  complex  apparatus  which  serves  to  intensify 
or  modify  the  original  stimulus.    The  sense  organ  determines 
the  kind  of  stimulus  which  may  be  received,  but  the  central 
nervous  organ  determines  the  nature  of  the  sensation  which 
results,  and  the  response. 

9.  Organs  with  different  origin  which  by  reason  of  similar 
function  have  come  to  look  alike  are  said  to  be  analogous. 
Organs  with  similar  origin  and  structure,  even  though -they 
may  appear  differently  because  of  their  differing  functions, 
are  homologous. 

112.  Topics  for  Investigation,  in  Library  and  Field. 

1.  What  are  the  advantages  and  the  disadvantages  of  divis- 
ion of  labor  and  differentiation?    Illustrate  your  views  very 
fully. 

2.  Illustrate  the  variety  of  foods  used  by  different  animals 
with  which  you  are  acquainted.     Classify  the  animals  you 
know  on  the  basis  of  their  food  preferences. 

7 


82  ZOOLOGY. 

3.  Compare  the  ways  in  which  animals  known  to  you  cap- 
ture and  prepare  their  food  for  swallowing.     What  special 
structures  arise  in  connection  with  this  function? 

4.  Do  animals  have  any  power  of  storing  food  within  the 
body  for  future  use?   Compare  with  plants. 

5.  Compare  gills  and  lungs  as  to  general  form  and  arrange- 
ment and  see  in  what  ways  they  appear  to  you  to  be  suited  to 
their  particular  media,  i.  e.,  gills  to  water  and  lungs  to  the  air. 
Why  might  not  the  conditions  be  reversed? 

6.  What  seem  to  you  to  be  the  comparative  advantages  and 
disadvantages  of  the  exoskeleton  and  endoskeleton. 

7.  Devices  to  accomplish  locomotion  in  animals  known  to 
the  student.     Find  as  many  variations  as  possible. 

8.  Select  four  animals,  as  diverse  as  possible,  representing 
each  of  the  following  conditions  of  locomotion : — through  the 
air,  through  the  water,  on  the  earth,  and  through  the  soil. 
Compare  the  problems  which  each- must  solve,  and  the  organs 
by  which  the  work  is  accomplished. 

9.  Compare  known  animals  as  to  rate  of  locomotion.     Do 
you  find  a  satisfactory  explanation  in  any  case? 

10.  Let  the  student  attempt  to  prove  that  the  dog  experi- 
ences the  same  sensations  which  we  have.     Hold  him  rigidly 
to  his  evidence. 

11.  Report  on  the  general  differences  between  the  eyes  of 
insects  and  of  vertebrates,  with  a  statement  of  their  structure 
and  the  work  done  by  each. 

12.  In  what  way  could  the  otocysts  possibly  act  as  organs 
to  enable  the  animal  to  appreciate  its  position  in  space,  and 
thus  maintain  its  equilibrium? 

13.  What  are  the  simpler  facts  connected  with  the  process 
of  absorption  or  osmosis  of  dissolved  substances  in  the  body? 

14.  Find  in  text-books  of  chemistry  a  fuller  account  of  the 
process  of  oxidation  and  why  it  results  in  a  liberation  of 
energy. 

15.  Demonstrate  how  a  biconvex  lens  forms  an  image  of 
objects.     Why  inverted? 


CHAPTER    VII. 

PROMORPHOLOGY. 

113.  We  have  seen  in  the  preceding  chapters  how  the  work  which  an 
organism  must  do  is  divided  among  its  parts,  and  that  the  parts  become 
specialized    in    connection    with    this    division   of    labor.     This   complexity 
which  is   known   as   organisation  is,   in   any  animal,  the  result  of  forces 
both  within  and  without  the  animal,  and  expresses  the  adaptation  of  the 
internal  structures  to  each  other  and  to  external  conditions.     The  simplest 
organism  known  is  thus  organized.     Organization  is  merely  more  evident 
in  the  more  complex  organism.     In  addition  to  the  fact  of  the  organiza- 
tion and  heterogeneity  of  structure  it  is  easy  to  see,  after  examining  a 
number  of  animals,   that   these   different   parts   are   not   thrown   together 
without  some  definite  order.     For  example,  the  ordinary  vertebrates  move 
with  their  long  axis  horizontal,  and  possess  certain  organs  that  we  always 
expect  to   see   at  the   anterior   end;   their   appendages   are  arranged   in   a 
definite  way  in  relation  to  the  long  axis.     The  parts  of  the  starfish  are 
arranged  according  to  a  different  but  equally  definite  plan.     All  considera- 
tion of  the  general  plan  according  to  which  animals  are  constructed  be- 
longs to  Promorphology.     The  fundamental  plan  may  be  similar  in  groups 
of  animals  which  are  otherwise  very  different,  because  of  similar  external 
conditions  and  similar  modes  of  life. 

114.  Definition  of  Sections. — In  trying  to  express  the  plan  of  struc- 
ture in  animals  it  is  convenient  to  have  in  mind  certain  planes  to  which 
we  can  refer  the  parts.     A  section  perpendicular  to  the  main  axis  of  an 
organism   or  of  an   organ   is   called   a   transverse  or   cross   section.     The 
longitudinal  median  section  separating  the  body  into  right  and  left  halves 
is  a  sagittal  section.     A  longitudinal  section,  perpendicular  to  the  sagittal 
and  separating  the  dorsal  (or  back)  and  ventral  (or  belly)  portions  of  the 
body  is   described   as   a  frontal  section.     An   animal   is   said  to  be   sym- 
metrical with  regard  to  any  of  these  planes  when  the  parts  severed  by 
the  plane  are  essentially  similar. 

115.  Axiality. — As  an  organism  grows  from  its  small  beginnings  in  the 
fertilized  ovum,  or  from  a  spore  in  the  simpler  forms,  the  new  materials 
may  be  added  more  or  less  uniformly  so  that  a  mere  increase  in  size 
results ;   or  growth  may  take  place  more   rapidly  along  some   radii  than 
along  others,  making  it  depart  from  its  original  spherical  form;  or  mate- 
rials or  organs  of  one  kind  may  occur  along  one  radius  and  different  ones 
on  another  (as  in  Fig.  46).     These  lines  of  special  growth  and  develop- 
ment are  called  axes.     We  may  investigate  them  as  to  their  number,  their 
space-relations  to  each  other,  and  the  likeness  or  unlikeness  of  the  two 
ends  or  poles  of  each  axis. 

83 


84  ZOOLOGY. 

116.  Types  of  Symmetry.— It  is  desirable  to  distinguish  the  follow- 
ing types: 

i.  In  a  spherical  organism  in  which  no  differentiation  is  apparent  (as 
in  a  simple  spherical  cell,  or  blastula,  Figs,  n  A,  3;  44)  any  plane  passing 
through  the  centre  divides  it  into  symmetrical  parts  and  all  the  axes  are 
essentially  equal.  In  such  a  case  there  may  be  said  to  be  an  infinite  num- 
ber of  similar  axes,  and  the  poles  of  each  axis  are  similar. 

FIG.  44. 


ec. 


FIG.  44.  Spherical  cell  (resting  stage  of  Amoeba)  illustrating  general  or  universal 
symmetry.  Any  plane  passing  through  the  centre  will  divide  it  into  two  essentially 
equal  portions. 

Question  on  the  figure. — -What  prevents  this  animal  being  a  perfect 
illustration  of  universal  symmetry? 

FIG.  45.     Amoeba  in  active  condition.     Entirely  unsymmetrical. 

2.  An    organism    may    be    wholly    asymmetrical,    without    any    definite 
form,  the  axes  being  without  regular  arrangement.     {Amoeba  in  its  active 
stages,    Fig.   45;    some   Sponges.)      In   other   instances   the   form   may  be 
definite  and  axes  developed,  but  the  structure  is  such  that  no  plane  will 
separate  the  animal  into  symmetrical  parts   (Paramecium  and  many  other 
active  Protozoa;  see  Figs.  66-69). 

3.  As  a  variation  of  the  universally  symmetrical  condition  seen  in   i, 
a  limited  number  of  axes  may  become  distinguishable  from  the  others  by 
some  specialization  of  structure   (Fig.  46).     These  special  axes  are  sim- 
ilar and  their  two  poles  are  alike. 

4.  Starting  again  from  the  undifferentiated  spherical  form,  one  of  its 
numerous  similar  axes  may  come  to  differ  from  all  those  perpendicular 
to  it  by  increased  or  diminished  length,  or  by  a  difference  in  construction. 
This  special  axis  is  to  be  known  as  the  principal  axis.     The  poles  of  the 
principal  axis  do  not  usually  remain  alike.     Perpendicular  to  this  princi- 
pal axis  one  may  select  an  indefinite  number  of  subordinate  axes  which 
are   essentially  similar   to   one   another.     The  poles  of   each   subordinate 
axis  are  alike.     Such  a  condition  is  realized  in  the  simplest  gastrulae  (Fig. 


PROMORPHOLOGY. 


ii ;  A  4,  B  3,  C  2).  Any  plane  including  the  principal  axis  will  divide  such 
an  organism  into  two  equal  halves.  In  general  external  appearance  a  hen's 
egg  would  illustrate  the  type.  This  is  the  least  differentiated  form  of  what 
is  known  as  radial  symmetry. 

FIG.  46. 


FIG.  46.  Actinomma  asteracantliion,  a  Radiolarian  with  a  limited  number  of  special- 
ized radii  (axes),  symmetrically  arranged  about  the  centre.  A,  whole  animal  with 
portion  of  two  spheres  of  shell  removed.  B,  section,  showing  relation  of  the  proto- 
plasm to  the  skeleton,  n,  nucleus;  p,  protoplasm;  sk.,  skeleton.  From  Parker  and 
Haswell. 

Questions  on  the  figure. — In  what  way  does  this  species  differ  in 
symmetry  from  Fig.  44?  How  many  specially  developed  axes  appear  to  be 
present?  By  how  many  planes  may  the  organism  be  divided  into  essen- 
tially equal  portions? 

Two  important  variations  from  this  simple  condition  of  radial  sym- 
metry are  found  in  the  animal  kingdom: 

(a)  Special  organs,  such  as  those  of  locomotion  and  t,he  like,  may  be 
developed  about  the  principal  axis.  These  usually  come  to  be  arranged 
in  a  limited  number  of  the  planes  which  may  be  passed  through  the  prin- 
cipal axis.  Considered  from  the  point  of  view  of  the  subordinate  axes 
this  means  that  there  are  a  limited  number  of  special  axes  (Fig.  47,  aa1 
and  bb1)  perpendicular  to  the  principal  axis  (Fig.  47,  o-ab.o)  instead  of 
an  indefinite  number  as  in  the  former  case.  These  special  subordinate 
axes  are  usually  3,  4,  or  5,  or  some  multiple  of  these  numbers.  The  num- 
ber however  may  be  reduced  to  two  in  which  the  four  poles  are  all  alike. 
Many  of  the  medusae,  coral  polyps,  and  some  echinoderms  illustrate  this 
type  of  symmetry. 

(fc)  A  further  variation  of  (a)  is  seen  in  the  fact  that  in  some  ani- 
mals, otherwise  similar  to  those  described  in  (a),  one  of  these  special 
axes  perpendicular  to  the  principal  axis  comes  to  differ  from  the  other. 


ab.  o. 


FIG.  47.  Diagram  of  Medusa,  illustrating  radial  symmetry.  A,  viewed  from  the 
oral  end  of  the  principal  axis;  B,  a  section  along  the  principal  axis  and  through  one  of 
the  subordinate  axes  ad1:  o,  ab.  o,  the  oral  and  aboral  poles  of  the  principal  axis;  a,  a1, 
and  b,  b1,  the  similar  poles  of  the  two  chief  subordinate  axes. 

Questions  on  the  figure. — Are  the  poles  of  the  oral-aboral  axis  alike 
or  unlike?  How  many  clearly  differentiated  secondary  axes  are  there? 
What  would  be  the  appearance  of  a  section  midway  between  aa1  and  bb1? 
Would  the  resulting  halves  be  symmetrical?  Compare  this  condition  with 
the  definition  of  radial  symmetry  in  the  text.  Find  other  illustrations  of 
radial  symmetry  in  the  figures  of  this  book. 

The  two  poles  of  each  of  the  subordinate  axes  are  essentially  alike,  but 
are  unlike  the  poles  of  the  other  subordinate  axis.  This  arrangement  is 
found  in  the  sea-anemone  (Fig.  48).  The  differences  between  aa1  and 
bb1  are  not  usually  so  great  that  we  cease  to  speak  of  the  form  as  radially 
symmetrical.  It  is  of  importance  to  know  that  in  the  radially  symmetrical 
animals  the  principal  axis,  whether  longer  or  shorter  than  the  subordinate 
axes,  is  normally  perpendicular  to  the  substratum  on  which  the  animal 
rests,  or  to  which  it  is  attached. 

5.  If  such  an  animal  as  was  last  described  were  to  have  its  principal 
axis  horizontally  placed,  with  one  of  its  two  subordinate  axes  vertical 
and  the  other  horizontal,  and  were  maintained  in  this  position,  it  would 
likely  happen  that  the  formerly  similar  poles  of  the  new  vertical  axis 
would  become  unlike,  because  subjected  to  different  influences.  These 
poles  are  known  as  the  dorsal  and  ventral  poles.  The  poles  (right  and 
left)  of  the  other  transverse  or  subordinate  axis  would  remain  similar, 
as  they  are  subjected  in  the  long  run  to  similar  conditions.  This  gives 
us  the  condition,  found  in  all  the  higher,  free-moving  animals,  known  as 
bilateral  symmetry.  It  consists,  to  recapitulate,  of  (i)  a  main  axis 
(antero-posterior  axis}  usually  horizontal  and  with  dissimilar  poles;  (2) 
a  transverse  axis,  usually  vertical  {dor  so -ventral  axis)  with  dissimilar 
poles;  and  (3)  a  transverse  axis  perpendicular  to  the  other  two,  hori- 


PROMORPHOLOGY.  8  7 

zontally  placed,  with  poles  alike  (right-left  axis).  Such  an  animal  has 
only  one  plane  (the  sagittal)  by  which  it  may  be  divided  into  symmetrical 
halves  (Figs.  49,  29,  96). 

117.  Antimeres.— There    is    a    striking   tendency    among   organisms   to 
repeat  or  duplicate  organs  or  parts.     This  we  have  seen  in  the  occurrence 

FIG.  48. 


m 


ab.  o. 

FIG.  48.  Diagram  of  the  sea-anemone,  illustrating  another  type  of  radial  symmetry. 
A,  cross  section;  B,  longitudinal  section.  Lettering  as  in  Fig.  47.  c,  the  chamber  be- 
tween the  mesenteries  (m). 

Questions  on  the  figure. — How  does  the  symmetry  of  the  anemone 
compare  with  that  of  the  Medusa?  Express  the  difference  clearly  in  terms 
of  the  axes  and  their  poles.  Are  aa1  and  bb1  strictly  similar  axes?  Do 
their  planes  divide  the  animal  into  halves?  Are  the  four  halves  thus 
obtained  equivalent?  In  B  what  difference  in  the  position  of  the  section 
will  account  for  the  differences  on  the  right  and  left  side  of  the  figure? 

of  similar  rays  about  the  main  axis,  in  radially  symmetrical  animals  like 
the  starfish.  Parts  thus  repeated  are  known  as  antimeres.  The  term  is 
also  applied  to  the  right  and  left  or  paired  halves  of  bilaterally  symmet- 
rical animals. 

118.  Metameres. — When  the  parts  or  organs  are  repeated  in  a  linear 
sequence  along  the  main  axis,  as  in  the  segments  or  rings  of  the  Earth- 
worm, the  arrangement  is  described  as  segmental  or  metameric.  Metam- 
erism may  be  shown  both  by  the  internal  and  external  structures.  The 
vast  majority  of  the  elongated,  bilaterally  symmetrical  animals  are  seg- 


88 


ZOOLOGY. 


mented.     In  the  higher  Vertebrates   it  is  not  manifest  externally,  but   is 
shown  in  the  vertebrae,  the  nerves,  etc. 

119.  Appendages. — Nearly  all  the  animals,  whatever  the  fundamental 
symmetry  may  be,  have  appendages  of  one  kind  or  another  for  locomo- 
tion, capture  of  food,  protection,  respiration,  and  the  like.  These  out- 
growths from  the  body  may  be  generally  distributed  over  the  body  surface 
(as  cilia  in  some  Protozoa  and  free-swimming  larvae)  ;  or  radially  ar- 
ranged,— often  about  the  mouth,  as  in  many  radially  symmetrical  animals 
(Figs.  47,  48,  79)  ;  or  in  a  right  and  left  series  in  bilaterally  sym- 

FIG.  49. 


FIG.  49.  Diagram  of  the  cross  section  of  a  fish,  showing  the  bilateral  symmetry  of 
the  parts:  dv,  dorsoventral  axis;  rl,  right-left  axis,  a.p.,  anterior  appendage;  b.c.,  body 
cavity;  ch,  notochord;  d.f.,  dorsal  fin;  g,  gut;  h,  heart;  h.a.,  haemal  arch;  m,  muscles; 
n.a.,  neural  arch;  sp,  spinal  cord;  v.c.,  vertebral  column. 

Questions  on  the  figure. — In  what  respects  is  the  symmetry  as  shown 
in  this  cross-section  different  from  that  shown  in  the  cross-section  of  sea- 
anemone?  Compare  carefully,  and  express  your  conclusions  in  terms  of 
the  axes  and  their  poles.  Find  other  figures  in  this  book  illustrating 
bilateral  symmetry. 


PROMORPHOLOGY.  89 

metrical  animals  (Figs.  129,  135,  188).  The  paired  appendages  of  bilat- 
eral animals  may  be  attached  dorsally,  laterally,  or  ventrally,  as  deter- 
mined by  the  uses  they  serve.  They  may  be  uniformly  distributed  along 
the  axis,  one  or  more  pairs  to  each  metamere,  as  in  some  arthropods,  or 
be  confined  to  special  segments  in  certain  regions  of  the  body,  as  in  the 
higher  arthropods  and  the  vertebrates. 

120.  Practical  Exercises. — Let  the  student  find  illustrations,  from  the 
animals  with   which   he  is   acquainted,   of  paired   appendages   with  dorsal 
attachment;  with  lateral;  with  ventral.     What  is  the  work  to  be  done  by 
each?     Does  their  position  appear  to  be  of  advantage  in  the  performance 
of  it?     Find  likewise  animals  in  which  the  appendages  are  clustered  at 
the  anterior  portion  of  the  body;  some  in  which  only  posterior  appendages 
are  found,  or  at  least  are  better  developed  than  the  anterior.     Does  the 
arrangement  seem  in  any  way  related  to  the  habits  and  surroundings  of 
the  animal  ? 

121.  Specialization  of  Metameres. — In  the  lowest  segmented  animals, 
as  worms,  the  metameres  are  much  alike  in  external  form,  in  their  ap- 
pendages, and  in  the  contained  structures.     In  the  adult  insects  this  be- 
comes less  true,  and  the  various  segments  are  specialized  for  particular 
duties.     The  segments  in  the  head  region  become  very  different  from  the 
body  segments.     The  same  is  even  more  true  of  the  higher  vertebrates. 
This   progressive    differentiation   of   a   distinct   head   is   one   of   the   most 
remarkable  facts  to  be  noted  in  animal  development.     Accompanying  the 
specialization  of  groups  of  segments  in  various  parts  of  the  body  we  often 
see  the  complete  fusion  of  such  similar  segments  for  the  better  perform- 
ance of  their  common  work   (as  in  the  head  of  insects  and  vertebrates, 
the  thorax  in  insects  and  Crustacea). 

122.  Formation  of  New  Segments  and  Regeneration. — In  many  of 
the  animals  in  which  the  segments  seem  of  nearly  equal  value  there  is  a 
more  or  less  continuous  formation  of  new  segments.     By  this  process  the 
organism   increases   in   length   and   in   the   number   of   its    segments,    and 
frequently,   with   the   aid   of   a   somewhat   similar  process,   produces  two 
individuals  by  division,  as  in  many  worms.     Such  a  proceeding  necessi- 
tates the  formation  of  a  new  head  or  tail  in  each  of  the  daughters,  by  a 
segment  which,  in  the  mother,  was  a  body  segment.     When  such  an  animal 
is   artificially  cut  in  two,   each  half  may  reproduce  segments   like  those 
which  have  been  removed  from  it.    This  is  known  as  regeneration.     Nat- 
urally this  ceases  to  be  possible  in  animals  in  which  the  segments  become 
more  highly  specialized;  yet  even  in  the  highest  animals  some  power  of 
replacing  lost  tissue  or  even  lost  organs  remains  (as  in  healing  of  wounds, 
formation  of  a  new  tail  by  lizards,  etc.).     It  is  recognized  as  a  general 
law  that  in  making  these  repairs  or  healings  the  newly-formed  material 
tends  to  restore  the  symmetry  possessed  at  the  outset. 

123.  Summary. — 

i.  Promo rphology  treats  of  the  ground  plan  in  accordance  with  which 
the  parts  of  animals  are  combined. 


9°  ZOOLOGY. 

2.  Symmetry  relates  to  the  possibility  of  passing  one  or  more  planes 
through  the  animal  and  obtaining  similar  portions  on  either  side  of  these 
planes. 

If  no  such  division  is  possible  the  organism  is  described  as  without 
symmetry.  If  each  of  three  mutually  perpendicular  planes  separates  the 
animal  into  equivalent  portions,  it  may  be  described  as  universally  sym- 
metrical. If  no  plane  transverse  to  the  main  axis  can  divide  the  animal 
into  symmetrical  parts,  and  two  or  more,  which  split  the  animal  along 
the  main  axis,  are  capable  of  doing  so,  we  describe  the  form  as  radially 
symmetrical.  When  there  is  only  one  such  plane  capable  of  separating 
the  body  into  equal  parts  we  have  the  condition  of  bilateral  symmetry, 
which  represents  the  highest  condition  of  development,  that  of  active 
animals. 

4.  Antimeres  are  parts  of  animals  repeated  on  different  sides   (two  or 
more)  of  the  main  axis  of  the  body. 

5.  Metameres   are  parts   repeated  in  the  main  axis,  i.   e.,  one  behind 
another.     The  successive  metameres  may  be  almost  entirely  alike  (homon- 
omous) ,  or  they  may  become  much  differentiated  in  the  performance  of 
diverse  functions  (heteronomous) . 

6.  From  the  main  trunk  of  animals  special  appendages  often  appear. 
They  usually  adapt  themselves  to,  and  accentuate,  the  fundamental  sym- 
metry of  the  organism.     They  may  therefore  be  asymmetrically  placed, 
or  uniformly  distributed  over  the  entire  surface,  or  along  the  radii,  or  in 
pairs  as  in  the  bilaterally  symmetrical  forms.     There  are  typically  one  or 
more  pairs  to  each  metamere,  though  this  number  may  be  much  reduced. 
Paired  appendages,  in  series,  are  regarded  as  homologous. 

7.  Many  animals  have  the  power  of  restoring  by  growth  parts  or  seg- 
ments   which   have   been   lost    (regeneration).     In   the    lower    segmented 
forms  this  power  is  closely  associated  with  the  power  of  increasing  the 
number  of   new   segments   in   an   uninjured   animal.     In   heteronomously 
segmented  animals  both  these  powers  are  less  manifest. 

124.  Topics  for  Investigation. 

1.  Determine  the  nature  and  degree  of  symmetry  in   (i)    the  sponge 
of  commerce;  (2)  skeleton  of  starfish;  (3)  crayfish  or  grasshopper. 

2.  What  is  the  final  criterion  by  which  you  determine  which  is  the 
anterior  and  which  the  posterior  end  of  an  animal?     Justify. 

3.  Find   among  animals   of  your   acquaintance   instances   of  difference 
between  the  dorsal  and  ventral  surfaces  as  to  color,  form,  etc.,  and  see 
if  you  can  discover  any  possible  advantage  resulting  therefrom. 

4.  What  degree  of  difference  have  you  ever  noticed  between  the  right 
and  left  halves  of  the  body  in  various  animals?     Is  perfect  bilateral  sym- 
metry ever  found? 

5.  Can  you  assign  any  reason  for  the  location  of  the  sense  organs  at 
the  anterior  end  of  the  body?     (Distinguish  between  cause  and  advan- 
tage.)    Do  you  think  they  occur  here  because  it  is  anterior,  or  is  it  ante- 
rior because  they  occur  here? 


PROMORPHOLOGY.  QI 

6.  Why  are  vehicles   (made  by  man)   bilaterally  symmetrical? 

7.  Express   in  tabular   form  the  differences  between  the  poles   of  the 
three  axes  of  bilateral  animals,  and  what  you  can  gather  as  to  the  causes 
of  the  differences. 

8.  For  what  kind  of  life  does  bilateral  symmetry  fit  the  animal? 

9.  For  what  kind  of  life  does   radial  symmetry  seem  suited?     Verify 
by  illustrations. 

10.  To   what  extent   do  animals   seem  able   to   regenerate  lost  parts? 
Trace  out  the  conditions  in  various  groups,  as  far  as  your  reference  library 
will  allow. 

11.  Is  there  any  apparent  limit  to  the  numbers  of  metameres  which 
animals  may  possess?     What  degree  of  constancy  or  variability  in  this 
particular  can  you  discover  in  various  individuals  of  the  same  species? 


CHAPTER    VIII. 

INDIVIDUAL  DIFFERENTIATION  AND  ADAPTATION. 

125.  The  Individual  and  its  Environment. — We  have 
thus  far  considered  the  mature  individual  as  the  end  for  which 
the  various  developmental  processes  exist.  It  has  been  seen 
that  the  individual  becomes  complex  as  its  parts  grow  and 
become  differentiated  to  do  the  work  necessary  to  the  well- 
being  of  the  animal.  In  this  differentiation  the  parts  become 
dependent  upon  each  other,  and  in  the  healthy  state  they  work 
harmoniously  among  themselves.  It  is  now  necessary  to  pass 
from  the  consideration  of  these  internal  structures  and  rela- 
tions in  order  to  consider  the  individual  animal  as  a  unit  in  its 
relations  to  everything  about  it,  that  is,  to  its  environment.  This 
term  includes  not  merely  the  inanimate  materials  and  condi- 
tions surrounding  an  organism,  but  in  addition  all  living  things 
both  plant  and.  animal  which  directly  or  indirectly  influence  it. 
The  environment  of  no  two  animals  is  the  san^  nor  is  it  the 
same  for  any  given  animal  for  two  moments  in  succession. 
This  continual  change  in  the  environment  leaves  its  impress 
on  the  structure  and  habits  of  all  organisms.  Every  individual 
is  thus  related  to  its  own  environment  from  day  to  day;  in 
addition  to  this,  all  the  individuals  of  any  generation,  owing 
to  the  facts  of  reproduction,  are  also  to  be  considered  in  the 
light  of  the  conditions  to  which  their  parents  have  been  sub- 
jected from  the  remotest  time.  The  study  of  the  individual  in 
its  relations  to  its  environment  brings  the  student  face  to  face 
with  many  very  important  problems.  No  department  of 
zoology  is  more  interesting. 

126.  Heredity. — One  does  not  study  organisms  very  long 
without  being  impressed  with  two  things :  first,  that  there  are 
remarkable  similarities  among  them,  even  among  those  little 

92 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.  93 

related;  and  second,  that  there  are  interesting  differences 
among  even  those  whose  kinship  would  entitle  them  to  great 
likeness.  There  is  always  a  disposition  among  students  to 
feel  that  the  likenesses  are  due  to  internal  causes  and  that  the 
unlikenesses  are  due  in  some  way  to  the  varying  external  in- 
fluences. In  other  words  the  former  are  thought  to  be  due 
to  heredity  and  the  latter  to  the  environment. 

Characteristics  which  animals  receive  from  their  ancestors 
near  or  remote  are  described  as  hereditary.  We  ascribe  the  fact 
that  the  hen's  egg  produces  a  fowl  and  a  frog's  egg,  a  frog  to 
the  action  of  heredity.  No  less  is  the  repetition  in  the  child 
of  minute  parental  peculiarities  of  feature  and  form  a  fact 
of  inheritance.  While  these  likenesses  are  due  to  the  action 
of  the  internal  forces  of  heredity,  it  must  not  be  deemed  that 
heredity  is  a  purely  conservative  influence  in  the  life  of  the 
organism.  The  offspring  of  two  parents  may  inherit  entirely 
different  qualities  from  their  parents  and  thus  present  differ- 
ences among  themselves  due  solely  to  inheritance.  The  off- 
spring may  also  present  such  a  mingling  of  the  qualities  in- 
herited from  their  ancestors  as  to  possess  characteristics 
decidedly  new  to  any  of  them.  Thus  likeness  to  parents  and 
unlikeness  to  parents  may  equally  be  due  to  heredity.  It  at 
once  perpetuates  old  qualities  and  introduces  variations. 

It  was  formerly  considered  that  all  the  characteristics  which 
parents  possess  are  equally  subject  to  inheritance,  but  it  is 
now  denied  by  many  biologists  that  the  qualities  which  a  par- 
ent acquires  in  its  own  lifetime,  as  the  result  of  its  own  actions 
or  of  the  environment,  are  capable  of  being  transmitted.  It 
is  unquestioned  however  that  qualities  received  from  the  par- 
ents are,  under  favoring  circumstances,  capable  of  being  trans- 
mitted to  offspring. 

127.  The  Bearers  of  Heredity. — It  follows  from  the  fact 
that  the  adult  organism  is  produced  from  the  union  of  the  male 
and  female  elements  that  these  two  cells  are  in  some  way  en- 
dowed to  carry  the  parental  qualities.  There  are  strong  evi- 
dences that  the  chromatic  elements,  or  chromosomes,  in  the 


94  ZOOLOGY. 

nuclei  of  the  male  and  female  cells  are  the  material  structures 
by  means  of  which  transmission  is  effected.  The  chromosomes 
of  the  fertilized  ovum  are  contributed  equally  by  the  male  and 
female  elements,  and  they  are  the  only  structures  in  the  sperm 
and  ovum  which  are  apparently  equal  in  amount.  This  taken 
in  connection  with  the  fact  that  one  parent  does  not  have  any 
more  power,  on  the  average,  to  influence  offspring  than  the 
other  furnishes  a  basis  for  the  belief  that  the  chromosomes  are 
the  physical  basis  of  heredity.  Recent  investigations  tend  to 
show  that  the  male  and  female  chromosomes  retain  their  dis- 
tinctness and  are  equally  distributed  to  all  the  nuclei  of  the 
developing  embryo. 

128.  Library  Exercises. — The  student  may  increase  his  knowledge  of 
the  facts  of  heredity  by  endeavoring  to  find  answers  to  the  following  ques- 
tions.    What  is  atavism,  and  what  explanations  have  been  offered  for  it? 
Do  the  male  and  female  seem,  as  a  rule,  to  have  equal  power  of  trans- 
mitting their  individual  characteristics?     Cite  some  facts  tending  to  show 
that  the  nucleus  is  especially  concerned  in  transmitting  parental  qualities; 
that  the  chromosomes  are  instrumental  therein.     What  are  the  essential 
features  of  the  old   "  preformation "  hypothesis  to   account  for  the  fact 
that  an  adult  similar  to  the  parent  springs  from  an  egg?     Examine  some 
of  the  principal  theories  of  inheritance  :    Darwin's  "  pangenesis  " ;  Brooks' 
modification  of  it;   Weismann's  "continuity  of  germ-plasm,"  etc.     What 
is  Mendel's  law  of  inheritance?  -  ,   , 

129.  Variability. — Notwithstanding  the  fundamental  like- 
ness existing  between  parent  and  offspring,  and  among  the  off- 
spring of  common  parents,  no  two  individuals  even  among  the 
lowest  animals  are  exactly  alike.    This  fact  of  variation  is  only 
less  fundamental  than  the  fact  of  likeness.     Variation  among 
animals  appears  to  depend  upon  two  sets  of  considerations: 
(i)  the  physical  and  chemical  instability  of  the  protoplasm  of 
which  animals  are  so  largely  composed,  and  (2)  the  diversity 
of  the  environment  in  the  broadest  sense.    Through  the  inter- 
action of  these  two  influences,  even  if  all  individuals  were  alike 
at  the  start,  it  would  only  be  a  question  of  time  until  the  off- 
spring derived  from  them  would  present  noteworthy  differ- 
ences.    Such  differences  would  tend  to  increase  with  the  lapse 
of  time.    This  is  the  more  true  in  proportion  to  the  degree  in 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.  95 

which  variations  are  capable  of  being  transmitted  under  the 
influence  of  heredity.  There  is  no  known  limit  to  the  power 
of  organisms  to  vary. 

130.  The  Part  Played  by  the  Environment  in  Producing 
Variation  while  not  completely  understood  must  be  recognized 
as  very  real.     Even  though  much  stress  must  be  put  upon  the 
hereditary  complexity  and  instability  of   protoplasm  as  the 
source  of  variations,  it  is  evident  that  the  external  conditions 
serve  as  stimuli  to  produce  the  changes  on  the  inside.     For 
example,  it  is  a  matter  of  common  observation  that  the  quan- 
tity and  quality  of  food  greatly  influence  not  merely  the  rate 
of  growth  but  the  size  and  quality  of  the  adult  organism  as 
well.     Life  would  be  impossible  without  food,  oxygen,  water 
and  suitable  temperature.    Any  variation  in  these  conditions  at 
once  has  its  effect  upon  the  organism.    Experiment  shows  that 
the  varying  degrees  of  salinity  of  the  water  may  be  accom- 
panied by  striking  individual  differences  of  form  in  certain 
marine  animals.     Caterpillars  of  certain  butterflies  placed  in 
boxes  lined  with  differently  colored  papers  develop  pupae  with 
colors  harmonizing  with  those  of  the  boxes  containing  them. 
Colors  in  various  animals  are  intensified  or  changed  by  special 
foods  or  by  changed  temperature.     In  general  it  may  be  said 
that  changes  in  any  of  the  conditions  important  to  animal  life 
produce  some  change  or  variation  in  those  animals  subjected 
thereto.      Since  this  is   true,   it  becomes   inevitable  that  the 
various  individual  animals  on  the  earth  are  differentiated  from 
each  other  somewhat  as  was  seen  to  be  the  case  with  the  cells 
and  tissue  of  which  the  individual  itself  is  composed.     The 
following  paragraphs  trace  out  some  of  the  ways  in  which 
this  differentiation  of  individuals  takes  place,  the  relations  of 
the  various  organisms  to  each  other  and  to  the  environment. 

131.  The  Struggle  for  Existence. — All  animals   (with  a 
few  possible  exceptions  in  those  which  possess  chlorophyll) 
depend  ultimately  upon  green  plants  for  food,  those  which 
live  on  other  animals  no  less  than  those  which  use  plant  food 


96  ZOOLOGY. 

directly.  Only  a  limited  amount  of  vegetation  can  be  sup- 
ported by  the  earth  without  cultivation.  The  number  of  ani- 
mals therefore  which  can  find  a  livelihood  on  the  earth  is  in 
turn  restricted.  There  is,  however,  no  such  limit  of  the  powers 
of  reproduction,  either  among  plants  or  animals.  Any  pair 
of  organisms  if  unchecked  could  in  a  very  few  years  supply 
descendants  enough  to  populate  the  earth  up  to  its  full  powers 
of  support.  That  they  do  not  thus  multiply  at  a  geometric 
ratio  is  due  solely  to  the  influences  at  work  to  destroy  these 
descendants.  Any  group  of  organisms  will  hold  its  own  when, 
on  an  average,  a  pair  of  individuals  can  in  a  life  time  bring  to 
maturity  another  pair  to  take  their  place.  More  than  this 
means  conquest  of  new  territory;  less  than  this,  the  extinction 
of  the  group.  When  we  recall  that  all  organisms  have  this 
unlimited  power  of  reproduction,  it  is  easy  to  see  that  a  time 
must  soon  come  when  a  struggle  for  food  and  a  foothold  on 
the  earth  is  inevitable.  The  struggle  would  be  more  intense 
between  those  organisms  which  demand  the  same  kind  of  food, 
that  is,  among  kindred.  This  is  the  fundamental  struggle.  It 
would  be  complicated  by  the  fact  that  some  groups  of  animals 
prey  upon  others,  and  that  the  primary  conditions  of  life,  as 
water,  temperature,  etc.,  are  subject  to  striking  changes. 
These  facts  tend,  by  just  so  much  as  they  destroy  individuals, 
to  relieve  the  struggle  within  the  species,  and  to  introduce  new 
factors  which  give  great  variety  and  interest  to  the  life  prob- 
lems of  animals.  There  is  nothing  more  certain  than  that  this 
struggle  has  occupied  organisms  practically  from  the  begin- 
ning, and  all  our  explanations  of  present  conditions  must  take 
note  of  the  fact.  All  the  important  structures  and  activities 
of  animals  are  modified  by  this  competition  for  a  livelihood. 

132.  Library  Exercises. — The  student  should  be  invited  to  make  real 
to  himself  the  possibilities  of  a  geometrical  increase  as  applied  to  organ- 
-  isms.  Take  the  known  rate  of  increase  (that  is,  the  total  number  of 
descendants  in  an  average  lifetime)  of  a  number  of  common  animals  and 
determine  the  possible  living  descendants  in  a  specified  time.  Find  ref- 
erences concerning  infusoria,  insects,  fish,  man.  Have  you  any  observa- 
tions relating  to  the  reality  of  the  struggle  for  food  among  animals? 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.  97 

133.  Natural  Selection. — In  spite  of  this  power  of  repro- 
duction we  see  that,  on  the  average,  individuals  do  not  increase. 
The  earth  is  no  more  thickly  inhabited  by  animals  today  than 
it  has  been  for  countless  ages.     The  proportions  of  different 
animals  vary  now  and  again,  but  that  is  all.     Out  of  a  family 
of  one  hundred  young  individuals,  no  two  of  which  are  alike, 
striving  for  a  foothold,  ninety-eight  will  be  destroyed.    Which 
will  survive?   Barring  accidents  beyond  the  powers  of  any  of 
the  individuals  to  resist,  those  will  survive  which  possess  or 
acquire  some  quality,  structure,  or  habit,  suited  to  the  struggle 
in  which  they  find  themselves.     This  may  be  a  matter  of 
strength,  of  speed  in  eluding  enemies  or  capturing  prey,  of 
specially  acute  senses,  of  a  tendency  toward  concealment,  or 
any  one  of  a  thousand  things  calculated  to  fit  an  organism  for 
a  special  place  in  life.    It  is  not  necessary  to  suppose  that  these 
elements  of  fitness  exist  in  striking  degree  at  first.    The  strug- 
gle is  so  intense  that  even  the  slightest  handicap  may  mean 
the  destruction  of  the  individual.     This  elimination  of  the 
weaker  individuals  results  in  what  has  been  called  natural 
selection' through  the  "  survival  of  the  fittest."    The  hereditary 
qualities  thus  preserved  in  the  individual  are  subject  to  trans- 
mission by  heredity;  and  by  the  continuous  action  of  natural 
selection  and  heredity  through  a  long  series  of  generations 
these  elements  of  fitness  are  believed  to  accumulate,  and  thus 
animals  become  better  and  better  adapted  to  their  surroundings. 

134.  Artificial  Selection. — Since  man  has  been  on  the  earth 
he  has  been  a  most  potent  factor  in  the  environment  of  the 
other  animals.     He  has  helped  in  the  elimination  of  animals 
hurtful  to  his  interests;  has  domesticated  others  which  he  has 
deemed  useful,  thus  rendering  their  environment  highly  arti- 
ficial and  removing  from  them  the  struggle  for  existence  in 
certain  measure.     For  natural  selection  he  has  substituted  a 
conscious  selection  of  such  organisms  as  are  best  suited  to  his 
needs  or  fancies,  and  has  allowed  these  to  reproduce,  eliminat- 
ing the  others.     This  artificial  process,  which  obtains  results 


9&  ZOOLOGY. 

more  rapidly  than  the  natural,  has  given  rise  to  the  various 
breeds,  strains  or  races  of  dogs,  horses,  cattle,  fowls,  etc. 
By  means  of  this  selection  the  habits  and  dispositions  of  the 
domestic  animals  have  been  improved  as  surely  as  their  struc- 
ture. Their  power  of  self-support,  however,  has  been  so  ma- 
terially diminished  that  some  of  them  could  not  succeed  in 
finding  a  living  in  the  wild  state  under  ordinary  circumstances. 

135.  Practical  Exercises. — Are  there  any  domesticated  animals  whose 
species  is  represented  in  the  wild  state?     Compare  the  habits  and  general 
structure  of  some  of  the  domesticated  animals  with  that  of  their  nearest 
kin  among  wild  species.     How  many  species  of  domestic  animals  can  you 
enumerate?     From  what  groups  do  they  come?    Trace  the  history  and 
results  of  the  domestication  of  some  of  the  common  animals,  as  fowls, 
pigeons,    cats,    dogs,    etc.     Have    any    strictly    American    species    been 
domesticated  ? 

136.  The  Adaptation  of  Animals  to  their  Environment. 
-There  are  two  distinct  questions  of  importance  to  be  con- 
sidered in  connection  with  this  subject:    (i)  the  necessity  of 
the  adjustment  of  organisms  to  their  environment,  and  (2)  the 
means  by  which  this  adaptation  takes  place  in  the  individual 
and  becomes  fixed  in  the  species.     It  is  clear  that  the  limited 
food  supply  and  the  unlimited  powers  of  animals  to  reproduce 
result  in  a  struggle  for  food  among  the  animals  at  any  time 
occupying  the  earth  ( 132) .    This  struggle  is  not  merely  among 
the  animals  in  question,  but  is  in  reality  between  every  organ- 
ism and  its  whole  environment.     Extremes  of  heat  and  cold, 
drouth  and  famine,  and  numerous  changes  in  the  conditions 
of  life  make  it  absolutely  necessary  that  the  individual  shall 
have  some  power  of  adapting  itself  to  what  is  permanent  and 
what  is  changeable  in  its  environment.     What  are  the  means 
then  by  which  animals  that  are  not  completely  in  accord  with 
their  surroundings  may  become  so?    There  are  two  possible 
ways  in  which  this  may  come  about.     The  animals  may  mi- 
grate to  regions  where  the  conditions  are  naturally  more  favor- 
able to  their  well-being,  that  is  to  regions  for  which  they  are 
already  adapted.     As  a  matter  of  fact  this  is  known  to  be  a 
common  occurrence.     Animals  often  disperse  from  their  old 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.  99 

centre  of  multiplication  under  the  influence  of  hunger  or  un- 
favorable local  conditions.  They  are  often  assisted  in  these 
dispersals  by  such  natural  agencies  as  winds,  currents  of  water, 
and  by  other  animals.  If  the  migrating  forms  succeed  in  find- 
ing new  regions  suited  to  their  needs,  there  results  a  condition 
of  adaptation  between  organisms  and  their  respective  environ- 
ments, but  without  any  active  change  in  the  characteristics  of 
the  organism.  The  environment  itself  is  subject  to  continual 
change  and  there  are  too  many  barriers  in  the  way  of  universal 
migration  for  this  to  be  accepted  as  a  complete  explanation  of 
the  widely  observed  adjustment  of  animals  to  the  conditions 
which  surround  them. 

In  the  second  place  animals  may  become  suited  to  their  en- 
vironment by  variation,  without  migration.  There  is  no  ques- 
tion that  this  also  occurs  and  that  it  is  the  more  important 
factor  of  the  two.  It  has  been  shown  (129)  that  all  animals 
are  variable.  Students  of  biology  have  suggested  two  impor- 
tant ways  in  which  variations  may  give  rise  to  a  harmony 
between  the  organism  and  its  surroundings.  This  result  may 
take  place  through  natural  selection  (133).  According  to  this 
view  the  organisms  naturally  tend  to  vary.  The^changing  en- 
vironment stimulates  this  tendency  to  variation.  Out  of  a 
thousand  individuals  of  similar  parentage  there  will  be  numer- 
ous slight  differences  of  structure  and  physiological  qualities. 
Some  of  these  will  be  more,  and  some  less,  favorable  to  the 
environment.  In  the  struggle  those  will  be  eliminated  which 
for  any  reason  are  strikingly  unsuited  to  the  environment.  On 
the  other  hand  those  animals  whose  variations  are  most  in 
accordance  with  the  local  condition  will  persist  and  propagate 
their  kind,  tending  through  heredity  to  pass  on  to  their  off- 
spring the  qualities  which  enabled  them  to  adjust  themselves 
to  their  surroundings.  Thus  there  will  be  a  gradual,  ever 
increasing  adaptation  in  the  whole  species  of  which  they  are  a 
part,  by  natural  selection.  Occasionally  there  occurs  in  off- 
spring, from  the  action  of  the  environment  or  from  other 
causes,  a  sudden  and  considerable  change  from  the  parent 


100  ZOOLOGY. 

type.  Such  a  product  is  known  as  a  "  sport/'  It  is  quite  pos- 
sible that  natural  selection  may  seize  on  such  and  if  in  a  favor- 
able direction  preserve  and  increase  them.  In  such  cases  adap- 
tation might  take  place  with  great  rapidity,  instead  of  in  the 
gradual  way  described  above. 

As  contrasted  with  natural  selection  of  indefinite  variations, 
it  has  been  argued  that  the  immediate  effect  of  the  environment 
on  the  organism  and  the  efforts  of  the  organism  to  respond  to 
the  stimuli  of  the  environment  produce  in  the  organism  just 
such  definite  variations  as  will  tend  to  fit  it  for  its  surroundings. 
In  other  words  the  majority  of  the  variations  brought  about 
by  a  given  external  condition  are  definite  and  naturally  in  the 
direction  to  meet  the  necessities  of  the  case.  For  example,  cold 
stimulates  the  surface  cells  of  the  body  of  an  animal.  The 
immediate  response  of  the  nervous  and  nutritive  processes  in 
the  organism  are  such  that  the  surface  cells  take  on  greater 
activity  and  produce  materials  at  the  surface  of  the  body 
which  tend  to  protect  the  animal  from  the  ill  effects  of  the  cold. 
This  is  an  individual  variation.  To  become  effective  in  making 
the  species  better  adapted  to  the  environment  these  results 
must  be  handed  down  by  inheritance  to  the  next  generation. 
If  this  can  take  place  this  theory  would  go  a  long  way  toward 
explaining  how  adaptations  arise.  There  is,  however,  con- 
siderable doubt  whether  such  adaptations  acquired  in  the  life 
of  an  individual  can  be  transmitted  to  offspring.  If  this  cannot 
occur  we  are  thrown  back  upon  natural  selection  as  the  prin- 
cipal explanation  thus  far  offered  to  account  for  the  pro- 
gressive adaptation  of  animals  to  the  environment.  There  is 
no  reasonable  doubt  that  natural  selection  is  such  an  explana- 
tion. To  what  extent  it  is  assisted  by  other  factors  is  at  pres- 
ent uncertain.  It  will  be  assumed  in  the  following  pages  that 
it  is  the  most  important  known  factor  in  producing  adaptation. 

137.  Classification.— Since  the  environment  is  not  the  same 
at  any  two  places  on  the  earth  and  there  is  an  accumulation, 
from  generation  to  generation  in  animals,  of  those  features 
which  tend  to  bring  them  into  harmony  with  their  different 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.          IOI 

environments,  it  is  inevitable  that  the  animals  themselves  come 
to  be  very  diverse,  no  matter  how  similar  they  were  at  the 
outset.  In  the  discussion  of  them  it  therefore  becomes  neces- 
sary to  devise  some  means  of  expressing  the  degree  of  like- 
ness and  unlikeness  among  the  great  number  of  individual 
animals  existing  on  the  earth.  This  may  be  done  by  means  of 
an  appropriate  classification.  The  differences  of  structure  and 
function  may  be  superficial  or  fundamental,  but  it  must  be  re- 
membered that  all  these  differences  are  in  some  way  the  out- 
come of  the  history  of  the  organisms,  and  that  the  likenesses 
are  signs  of  kinship,  or  of  similar  history,  or  both.  The  group- 
ing or  classifying  of  organisms  has  two  objects:  (i)  con- 
venience, that  is,  to  make  future  work  easy;  and  (2)  to  express 
the  results  of  past  study.  Insomuch  as  the  first  motive  may 
predominate  the  classification  may  be  artificial,  that  is  may 
bring  together  animals  that  are  really  not  closely  related, 
though  possessing  a  superficial  resemblance.  The  grouping 
together  of  bats  and  birds  on  the  ground  of  their  power  of 
flying,  or  whales  with  fishes  because  of  their  habitat,  would 
illustrate  such  a  classification.  In  proportion  as  classification 
takes  in  all  the  facts  known  with  regard  to  animals  and  ex- 
presses the  relationship  of  forms  classed  together,  it  is  said 
to  be  natural.  Every  classification  is  in  some  measure  artificial 
since  we  do  not  know  all  the  facts  concerning  the  structure  or 
history  of  any  organism. 

138.  Terms  Used  in  Classification. — From  what  has  been 
said  concerning  the  power  of  multiplication  in  animals,  the  re- 
sulting struggle  for  existence,  the  variability,  and  the  elimina- 
tion of  those  whose  variations  are  not  suited  to  the  various 
environments  into  which  the  offspring  migrate,  it  will  be 
readily  understood  that  even  the  descendants  of  a  single  pair 
of  organisms  will  come  in  time  to  be  noticeably  different  in 
form,  size,  color,  and  the  like.  The  individuals  of  a  given 
region  will  usually  be  more  like  each  other  than  like  their 
cousins  who  have  been  subjected  to  some  other  kind  of  en- 


102  ZOOLOGY. 

vironment.  There  is  thus  a  need  of  terms  to  express  the  de- 
gree of  difference  which,  through  these  influences,  finally  char- 
acterizes the  descendants  even  of  common  ancestors.  Such 
groups  of  forms  are  usually  known  as  varieties  or  subspecies 
of  the  original  type  from  which  they  all  sprang.  Thus  in  the 
human  race  while  all  men  are  considered  as  belonging  to  one 
common  type  and  possibly  derived  from  the  same  human 
ancestors  there  is  enough  difference  between  the  American 
Indian  and  the  Caucasian  to  make  it  necessary  to  distinguish 
them  as  different  varieties.  Many  of  our  widely  distributed 
animals  as  the  dog,  the  horse,  the  common  fox  have  varieties 
which  are  readily  distinguishable.  When  the  causes  which 
produce  varieties  have  been  at  work  long  enough  to  eliminate 
the  intermediate  forms  which  are  often  found  connecting  the 
varieties,  and  to  secure  a  close  adaptation  of  the  varieties  to 
the  environment,  the  term  species  is  applied  to  what  were  for- 
merly called  varieties.  Species  thus  merely  represent  the  fur- 
ther progress  of  individual  differentiation  and  adaptation  to 
the  different  modes  of  life  which  give  rise  to  variation  in 
individuals — that  is,  to  varieties.  A  species  of  animals  may 
again  split  up  by  the  action  of  the  forces  mentioned  (and  other 
conditions  which  have  not  been  mentioned)  into  new  varieties 
and  finally  into  new  species.  It  is  believed  that  the  present 
diversity  of  animal  and  plant  life  has  come  about  from  a  much 
more  limited  number  of  kinds  of  ancestors  by  a  method  essen- 
tially such  as  that  described  above.  Varieties  of  the  same 
species  usually  cross  freely  and  the  offspring  are  known  as 
mongrels.  The  individuals  of  different  species  as  a  rule  cross 
less  freely  and  when  they  do  cross  their  offspring  are  called 
hybrids.  Hybrids  are  often  sexually  infertile. 

The  genus  is  related  to  species  somewhat  as  the  species  to 
the  varieties  which  compose  it.  A  genus  embraces  those  kin- 
dred species  which  show  a  high  degree  of  relationship  among 
themselves.  The  characters  which  serve  to  distinguish  differ- 
ent genera  are  more  fundamental  than  those  by  which  we 
recognize  varieties  or  species,  and  argue  a  more  extended  time 
in  the  differentiation  of  genera  than  is  required  for  species 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.         103 

Other  terms,  as  families,  orders,  classes,  phyla,  are  used  to 
denote  the  still  more  extensive  and  comprehensive  divisions 
of  the  animal  kingdom. 

139.  Illustration  of  Classification. — The  domestic  cat  has 
many  varieties  or  breeds,  as  the  maltese,  manx,  tortoise-shell, 
etc.  On  the  other  hand,  the  wild-cat,  the  tiger,  the  leopard, 
the  lion  have  numerous  points  of  structural  likeness  to  the 
domestic  cat,  and  are  said  to  be  species  belonging  to  the  same 
genus  (Fells}.  The  genus  Fells  and  others  less  common  are 
placed  together  in  the  family  Felldce.  These  with  the  members 
of  the  dog  family  and  others  constitute  the  order  Carnlvora 
(flesh  eaters),  and  similarly  for  the  higher  groups  in  the 
diagram  below. 

Kingdom — Animalia. 

Phylum — Chordata  (fishes,  birds,  mammals). 

Class — Mammalia  (carnivora,  ruminants,  bats,  man). 
Order — Carnivora  (dogs,  wolves,  cats,  etc.). 
Family — Felidae  (cat  family). 

Genus — Felis  (cat,  lion,  tiger,  etc.). 

Species — Fells  domestlca  (with  its  numerous 

varieties). 

The  name  of  an  animal  is  its  generic  name  followed  by  its 
specific  name  as  above.  The  variety  name  is  added  when  there 
are  distinct  varieties. 

146.  Relation  of  the  Individual  to  the  Species. — The  vari- 
ous types  of  animals  produce  their  offspring  in  numbers  pro- 
portional to  the  difficulties  encountered  in  bringing  the  young 
to  maturity.  In  the  most  favorable  circumstances  many  more 
are  produced  than  can^survive.  In  cases  where  enemies  are 
numerous  millions  of  eggs  may  be  deposited  in  order  to  secure 
a  single  adult.  Nature  is  thus  said  to  be  lavish  in  her  waste 
of  individuals  in  order  that  the  species  may  be  continued  and 
improved  in  its  adaptations.  These  surviving  descendants, 
generation  after  generation,  have  become,  through  natural 
selection,  more  and  more  suited  to  their  surroundings.  This 


104  ZOOLOGY. 

means  that  the  production  of  many  individuals,  a  large  num- 
ber of  which  never  reach  maturity,  secures  the  development  of 
a  small  aristocracy  which  propagates  the  type.  The  species 
is  related  to  its  individuals  -something  as  the  individual  is  to 
the  renewed  and  changing  cells  of  which  it  is  composed. 
Species  are  not  constant,  but  even  the  most  fixed  must  undergo 
change  or  extinction  when  confronted  by  new  conditions. 
Species  however  are  less  variable  than  the  individuals  com- 
posing them  because  the  species  represents  an  average  con- 
dition of  all  its  individuals.  Adaptation  to  environment  is  the 
great  problem  which  every  animal  must  solve.  Those  which 
do  solve  it  successfully  constitute  the  species.  It  is  needful 
then  to  consider  next  those  characteristics  of  structure,  habit, 
or  instinct  whereby  a  species  of  organisms  becomes  success- 
fully adjusted  to  its  surroundings.  In  a  broad  sense  all  the 
organs  which  were  outlined  in  the  preceding  chapters  are  adap- 
tations :  the  digestive  organ  and  process,  to  the  nature  of  food ; 
the  nervous  system  and  the  special  senses,  to  the  external 
stimuli;  the  lungs,  gills,  and  skin  to  the  need  of  oxygen,  and 
the  like.  In  contrast  to  adaptations  of  this  kind  we  now  con- 
sider as  adaptations  those  more  special  modifications  of  fun- 
damental structure  by  which  a  species  becomes  more  suited 
to  some  limited  habitat  or  to  some  special  mode  of  life  which 
is  of  signal  use  to  it  in  the  struggle  for  existence. 

141.  Classification  of  the  Principal  Types  of  Adaptation. 

A.  Adaptations  primarily  in  relation  to  the  inorganic  en- 
vironment. 

B.  Adaptations  primarily  related  to  other  organisms. 
I.  Among  animals  of  the  same  species. 

1.  Friendly  and  social, 
(a)   Mating. 

(fr)   Parental  care  of  young. 

(c)  Organic  colonies. 

(d)  Social  and  communal  life. 

2.  Competitive:  for  food,  mates,  etc. 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.         IO5 

II.  Among  animals  of  different  species. 

1.  Friendly  and  social. 

(a)  Commensalism. 

(b)  Symbiosis. 

2.  Competitive. 

(c)  The  predaceous  habit:  adaptations  for  offense 
and  defense. 

(d)  Parasitism. 

142.  Adaptations  to  the  Inorganic  Environment. — These 
embrace  such  special  structural  devices  as  hair,  feathers,  the 
blubber  of  whales,  which  enable  the  body  to  maintain  its  tem- 
perature despite  the  condition  of  the  medium.  The  habits  of 
burrowing  and  hibernation,  the  winter  migrations  of  many 
animals,  especially  birds,  are  examples  of  instinctive  adapta- 
tion to  cold.  The  same  end  is  obtained  by  man  by  artificial 
clothing,  by  houses,  and  by  the  use  of  fire  which  has  been  one 
of  the  most  important  instruments  in  his  progress.  Rotifers, 
infusoria,  and  some  other  animals  have  become  capable  of 
retaining  life  during  thorough  drying,  and  of  resuming  activ- 
ity on  the  return  of  moisture.  Adaptations  to  locomotion  in 
different  media,  earth,  air,  and  water;  to  climbing;  to  station- 
ary life,  belong  to  this  group.  These  are  only  a  few  of  the  many 
instances  of  adaptations  of  organisms  to  the  materials  and 
the  forces  about  them.  It  is  easy  to  see  that  some  of  the  adap- 
tations are  of  life  and  death  importance,  and  without  them  the 
species  would  become  extinct.  It  is  believed  that  these  qualities 
of  the  organism  arise  in  a  way  something  like  this :  owing  to 
the  irritability  of  all  protoplasm,  the  prevalent  external  factors 
as  heat,  light,  gravity,  moisture,  and  chemically  active  sub- 
stances must  produce  some  change, — that  is,  some  response 
on  the  part  of  the  organism.  Those  organisms  in  which  the 
response  is  not  in  accordance  with  the  best  adjustment  to  the 
special  environment  are  less  likely  to  survive  in  the  struggle 
for  existence.  Those  which  do  survive  and  propagate  their 
kind  because  of  their  favorable  responses  to  these  stimuli  are, 


io6 


ZOOLOGY. 


by  reason  of  these  facts,  more  and  more  likely  in  succeeding 
generations  to  possess  those  habits  and  structures  suiting  them 

to  their  surroundings. 

FIG.  50. 


FIG.   50.     Young  Opossum   (Didelphys  virginiana)   photographed  by  J.  W.   Folsom. 

Questions  on  the  figure. — Of  what  conceivable  value  to  the  animal 
is  the  prehensile  tail?  In  what  other  groups  of  animals  is  the  tail  pre- 
hensile? What  are  the  habits  of  the  opossum?  How  is  this  species  dis- 
tributed on  the  earth?  Where  are  other  marsupials  found? 

.  143.  Practical  Exercises. — Find  other  instances  which  seem  to  indicate 
adaptation  either  in  structure  or  habit  to  special  features  in  the  environ- 
ment: as  adaptations  to  prevent  undue  evaporation  in  a  dry  climate; 
adaptations  to  warm  conditions;  to  drouth;  to  the  use  of  special  plants 
as  food;  to  light;  to  gravity.  Illustrate  from  observation  and  by  library 
reference  the  types  of  adaptations  cited  in  the  text  above.  Is  the  power 
of  sleeping  an  adaptation  of  any  value  ?  Among  what  animals  is  it  found  ? 
Find  instances  in  which  useful  adaptations  have  become  useless  and  even 
hurtful  from  changed  conditions  of  life. 


INDIVIDUAL    DIFFERENTIATION   'AND    ADAPTATION.         IO7 

144.  The  relations  of  animals  of  the  same  species  to  one 
another  is  an  interesting  mixture  of  competition  and  coopera- 
tion.   In  the  higher  forms  the  parents  instinctively  make  great 
personal  sacrifices  that  the  offspring  may  be  cared  for;  the 
offspring  on  the  other  hand  struggle  with  each  other  for  this 
parental   provision.      In   the   classification   offered    (141)    it 
should  be  remembered  that  both  friendly  and  competitive  habits 
and  structures  are  always  represented  in  the  same  individual. 

145.  Mating    Adaptations. — One    of    the    most    striking 
forms  of  individual  variation  is  seen  in  the  differences  be- 
tween the  sexes  of  higher  animals.     The  male  and  female  are 
often  so  widely  different  in  form,  size,  color,  and  other  quali- 
ties, that  naturalists  have  classified  them  as  belonging  to  dif- 
ferent species  and  yet  it  is  very  manifest  that,  though  different, 
the  sexes  are  closely  adapted  to  each  other.    In  the  lower  types 
of  animals  the  sexes  are  frequently  represented  in  the  same 
individual.     In  such  cases  the  sexual  elements  often  mature  at 
different  times.    An  individual  is  thus  alternately  male  and  fe- 
male.   This  is  regarded  by  many  as  being  the  primitive  condi- 
tion,— the  separation  of  the  sexes  being  accomplished  by  the 
repression,  so  to  speak,  of  one  or  the  other  sex  in  each  individual. 
It  is  now  known  that  the  temperature  and  the  amount  and  qual- 
ity of  food  have  something  to  do  with  the  proportion  of  males 
and  females  which  are  produced.     So  sexual  dimorphism  is 
in  some  measure  a  response  to  external  conditions  and  presents 
every  evidence  of  being  an  advantageous  adaptation  to  the 
conditions  of  life.    The  very  union  of  the  sperm  and  the  ovum, 
whereby  two  cells  lose  their  individuality  in  one,  with  a  renewal 
of  powers  and  the  mingling  of  the  qualities  of  two  parents, 
must  be  looked  on  as  an  adaptation  of  the  very  highest  moment 
to  the  animals  in  which  it  first  appeared,  and  to  their  de- 
scendants.     The   chemical   attraction   which   the   female   cell 
exerts   on  the  motile  sperm  cell  is  a  special  adaptation  to 
accomplish  this  union.     Furthermore  it  is  undoubtedly  true 
that  many  of  the  color-markings,  notes,  motions,  and  the  like 


108  ZOOLOGY. 

in  which  the  male  and  female  animals  differ  are  recognition 
marks  whereby  the  presence  of  one  sex  is  made  known  to 
the  other.  In  some  animals  in  which  fertilization  normally 
occurs,  the  ova  may  develop  in  the  absence  of  sperm  (parthe- 
nogenesis}. This  may  have  arisen  as  an  adaptation  to  tem- 
porary scarcity  of  males.  This  view  is  in  some  cases  supported 
by  the  additional  fact  that  parthenogenetic  eggs  produce  male 
individuals  wholly,  or  in  excess. 

146.  Practical  Exercises. — What  is  the  difference  in  the  notes  of  the 
male  and  female  of  the  American  quail  which  would  serve  as  recognition 
marks?     Mention  other  cases  of  sexual  dimorphism  which  appear  to  you 
to  serve  a  similar  end.     What  evidences  have  we  that  the  mingling  of 
sperm  and  ovum  results  in  a  rejuvenescence?  in  the  introduction  of  greater 
variation?   in  conservatism?     Show   how   these  are   important  as   adapta- 
tions in  the  struggle   for  existence.     In  what  groups   is  parthenogenesis 
found?     Give  details  of  the  facts  in  several  cases. 

147.  Reproduction  and  Care  of  Young. — The  very  rate 
of  reproduction  is  an  adaptation  to  the  severity  of  the  strug- 
gle for  existence  experienced  by  the  animals  of  a  given  species. 
Those  forms  with  few  enemies  and  abundant  food  usually 
need  to  produce  only  a  few  young  in  order  to  maintain  their 
place.     Others  less  favored  in  these  regards,  as  many  insects, 
the  lobster,  the  salmon,  must  reproduce  thousands  of  young 
in  a  lifetime.     Similarly  the  length  of  the  reproductive  period 
and  of  life  becomes  an  adaptation  to  the  same  end.    It  is  clear 
from  these  facts,  that  any  device  which  the  parent  may  adopt 
likely  to  bring  a  larger  percentage  of  the  young  to  maturity 
will  make  for  a  saving  in  the  necessary  birthrate.     This  hus- 
bands the  parental  resources  and  conduces  to  the  efficiency  of 
the  individual  and  of  the  species.    It  must  not  be  supposed  that 
parental  care  is  confined  to  the  higher  animals.     In  its  most 
elementary  condition  it  takes  the  form  of  food  stored  in  the 
egg,  and  in  depositing  the  egg  in  a  safe  place  for  hatching. 
After  hatching  it  takes  the  form  of  supplying  food,  or  protec- 
tion, or  both.     Cephalopods,  fishes,  and  birds  have  a  large 
amount  of  food  substance  stored  in  the  egg.     Many  animals, 
as  the  clam,  some  fishes,  some  reptiles,  and  the  mammals,  re- 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.         109 


FIG.  51. 


FIG.   51.     Galls  on  oak,  cynipid  (Holcaspis  duricoria).     Natural  size.     Photo  by  Folsom. 


FIG.  52. 


FIG.  52.     Galls  on   elm,   produced  by   an   aphid,    Colopha  ulmicola.     Natural   size. 
Photo   by   J.   W.   Folsom. 


HO  ZOOLOGY. 

tain  the  eggs  in  special  portions  of  the  body  until  development 
has  well  begun.  The  flies  lay  their  eggs  in  the  decaying  matter 
which  the  young  use  as  food.  The  solitary  wasps  seal  theirs 
up  in  nests  with  the  food  (dead  or  wounded  spiders  or  insects) 
on  which  they  are  to  develop.  Other  insects  bore  into  the 
tissues  of  living  plants  and  deposit  their  eggs,  about  which 
"  galls  "  or  masses  of  abnormal  vegetable  tissue  are  developed. 
The  ichneumon  fly  deposits  its  eggs  in  the  body  of  some  other 

FIG.  53- 


FIG.   53.     Galls   on    hackberry   leaf,   produced   by   a  fly    (Cecidomyiidce).     Natural   size. 

Photo    by    Folsom. 

Questions  on  figures  51,  52,  53. — What  does  the  gall  represent  from 
the  point  of  view  of  the  plant?  From  the  point  of  view  of  the  insect? 
What  seems  to  cause  the  undue  vegetable  growth?  Find  other  galls  in 
nature  and  try  to  find  what  type  of  insect  is  responsible  for  them?  In 
what  ways  may  one  hope  to  determine  this  fact? 

animal.  Thus  we  see  an  immense  number  of  adaptations 
useful  to  the  organism  have  been  developed  in  connection  with 
the  egg-laying  habit.  After  such  provision  the  majority  of 
animals  leave  the  young  to  care  for  themselves;  but  many 
higher  forms  take  further  pains  to  protect  and  train  their 
offspring  during  the  course  of  their  development.  The  care 
which  the  birds  and  mammals  give  their  young  is  a  matter 
of  common  observation.  It  takes  the  form  of  food,  of  special 
homes, — as  nests,  burrows,  dens,  etc.,  and  of  the  personal  ser- 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.         Ill 

vices  of  the  parents,  who  will  often  at  the  risk  of  their  own 
life  protect  the  young  from  its  enemies.  Similar  care  is  shown 
by  some  insects,  especially  the  social  forms,  such  as  bees,  ants, 
and  the  like.  The  lobster  carries  its  young  on  its  abdominal 
appendages  for  months  after  hatching.  The  lower  Inverte- 
brates are  practically  destitute  of  these  later  care-taking 
instincts. 

It  is  interesting  to  notice  that  animals  differ  very  much  in 
their  helplessness  at  hatching  or  at  birth.     The  young  of  the 

FIG.  54. 


FIG.   54.     Nestling  Marsh  Hawks  (Circus  cyaneus).     From  Year-Book.     Department  of 

Agriculture. 

Questions  on  the  figure. — What  are  the  nesting  and  breeding  habits 
of  the  marsh  hawk?   Are  the  young  precocial  or  altricial? 

reptiles,  or  the  duck,  or  the  chicken  are  relatively  well  devel- 
oped at  hatching,  and  are  very  soon  able  to  run  about  and 
feed.  The  young  of  the  song  birds,  as  the  thrushes,  swallow^, 
etc.,  are  wholly  dependent  on  the  care  of  the  parents  for  a 
considerable  time.  In  the  herbivorous  mammals,  as  the  sheep 
and  cattle,  the  young  have  the  use  of  their  limbs  in  a  short 


1 1 2  ZOOLOGY. 

time.  Among  the  carnivorous  forms,  as  the  cat  and  dog,  the 
young  are  more  helpless.  In  the  human  species  the  period 
of  helplessness  is  longest  and  consequently  the  necessity  of 
parental  care  greatest.  In  general,  the  longer  period  of 
parental  protection  accompanies  the  development  of  more  com- 
plex and  highly  organized  instincts,  and  intelligence.  The 
lengthened  period  of  dependence,  while  a  burden  to  the  parent 
in  one  sense,  is  an  advantage  to  it  in  the  saving  in  number  of 
offspring,  and  serves  to  benefit  the  species,  not  merely  by  keep- 
ing the  offspring  alive  until  they  may  reproduce,  but  in  the 
greater  development  of  such  parental  instincts  as  gentleness, 
self-sacrifice,  and  the  like.  In  the  human  race  it  has  given  rise 
to  the  home  and  family,  which  we  regard  as  the  real  basis  of 
modern  society;  and  social  organization  in  turn  has  been  a 
most  powerful  factor  in  the  progress  of  the  human  species. 
Death  and  the  length  of  life  must  also  be  considered  as  special 
adaptations!  This  differs  in  different  species  very  widely. 
Life  in  general,  where  natural  selection  acts,  will  be  the  period 
of  youth,  plus  the  period  of  fertility,  plus  the  time  necessary 
to  rear  the  latest  offspring.  For  the  species,  the  death  of  the 
individual  becomes  an  advantage  at  the  completion  of  this 
period,  and  this  fact  is  sufficient  to  insure  that  death  will 
normally  occur  at  this  time. 

148.  Practical  Exercises. — Add  instances  of  parental  care  which  have 
fallen  under  your  own  observation,  and  give  a  statement  of  the  facts  in 
the  case.     Compare  the  mammals  with  which  you  are  acquainted  in  this 
regard.     Compare  the  condition  of  the  young  of  the  robin,  the  quail,  the 
blue-jay,  the  pigeon  as  to  maturity  at  hatching.     Do  any  animals  .of  your 
acquaintance  reproduce  more  than  once  in  a  year?    Why  is  one  reproduc- 
tive period  per  year  a  common  adaptation.     Compile  statistics  concerning 
the  longevity  of  various  animals,  and  its  relation  to  size,  to  reproductive 
period,  and  to  the  time  demanded  to  reach  the  adult  stage. 

149.  Colonies. — In  some  of  the  lower  groups  of  animals, 
as  the  polyps  and  jelly-fishes,  in  which  the  reproduction  by 
fission  or  budding  is  prominent,  the  newly-formed  individuals 
remain  for  a  longer  or  shorter  time  in  association  with  the 
parent,  or  with  each  other.    These  units  which  otherwise  might 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION  113 

be  separate  individuals  are  organically  connected  and  often 
come,  by  the  continuation  of  the  process,  to  form  immense 
masses,  as  in  the  coral.  Such  organic  associations  are  called 
colonies.  Colonies  rarely  occur  in  animals  in  which  the  organs 
are  highly  specialized.  Very  often  the  individuals  become 
specialized  for  the  performance  of  a  special  portion  of  the 
work,  and  thus  we  get  several  quite  differently  constructed 
individuals  within  the  colony  (polymorphism,  Fig.  85).  The 
whole  colony  may  then  behave  somewhat  as  an  individual,  the 
polyps  taking  the  place  of  organs  (Siphonophora).  Colonial 
animals  are  almost  always  attached  to  fixed  or  floating  objects. 
These  polymorphic  individuals  are  closely  adapted  to  each 
other  in  structure  and  division  of  labor ;  and  the  colonial  habit 
in  general,  even  where  there  is  no  division  of  labor,  is  a  suc- 
cessful device  whereby  limited  areas  are  completely  occupied 
by  the  members  of  a  species  (as  in  the  case  of  the  branching 
corals)  where  the  single  polyps  would  be  practically  helpless. 
The  arrangement  of  the  polyps  on  the  common  skeleton  and 
the  rate  of  growth  of  the  different  polyps  are  beautifully 
adapted  to  the  best  use  of  the  currents  of  water  by  which  the 
food  and  oxygen  are  conveyed. 

150.  Library   Exercise. — What  phyla  of  the   animal   kingdom   supply 
instances  of  organic  colonies?     Trace  different  degrees  of  polymorphism. 
In  what  different  ways  do  the  individuals  occur  on  the  common  stock? 
Show  how  the  relative  rate  of  growth  of  the  differently  placed  individuals 
determines  the  ultimate  form  of  the  colony  as  a  whole. 

151.  Social  and  Communal  Life. — Animals  of  the  same 
species  often  become  associated  even  when  there  is  no  organic 
connection  between  the  individuals.     The  association  may  be 
temporary  or  permanent.     In  its  simplest  form  this  is  merely 
a  matter  of  gregariousness  such  as  is  seen  in  the  schools  of 
fishes  or  flocks  of  birds,  which  are  apparently  brought  together 
at  certain  periods  by  a  common  instinct  or  by  common  needs. 
A  step  more  intimate  is  the  banding  together  of  predaceous 
animals  as  wolves  or  vultures,  or  pelicans,  for  mutual  help  in 
finding  or  capturing  the  prey.     Corresponding  to  this,  on  the 

9 


114  ZOOLOGY. 

part  of  their  victims,  we  find  the  herding  of  the  bison,  of  deer, 
and  their  allies  for  protection,  whether  by  fighting  together  or 
by  the  stationing  of  sentinels  to  give  notice  to  the  feeding  herd 
of  the  approach  of  danger.  In  still  other  forms,  notably 
among  such  insects  as  the  bees  and  ants,  there  is  a  very  intimate 
and  permanent  union  in  social  life.  This  is  usually  associated 
with  the  instinct  of  home  building,  and  thus  a  high  degree  of 
division  of  labor  with  its  great  advantages  becomes  possible. 
This  is  carried  to  such  an  extent  that  often  polymorphic  in- 
dividuals result,  much  as  in  the  organic  colonies.  In  such  cases 
it  is  clear  that  the  individual  life  comes  to  be  bound  up  in  the 
success  of  the  community.  Such  forms  usually  exert  great 
care  for  their  young  and  develop  a  relatively  high  order  of 
intelligence.  The  principal  social  forms  are  the  ants,  of  which 
there  are  more  than  two  thousand  species ;  some  of  the  bees 
and  wasps;  the  termites,  or  so-called  white-ants;  beavers; 
some  monkeys  and  man. 

152.  Library  Exercise. — Make  a  report  on  the  social  life  of  the  honey- 
bee, including  the  following  points :    the  home ;  the  kinds  of  individuals, 
their  origin,  and  their  work  in  the  community;  their  food  and  its  prepara- 
tion; mode  of  caring  for  the  young;  swarming  and  its  significance.     Make 
a  similar  report  concerning  some  species  of  ant.     Find  facts  concerning 
the  following  topics :    "  ants'  cows  " ;  slave-making  among  the  ants ;  army 
ants ;  the  agricultural  ant. 

153.  Competition  Among  Animals  of  the  Same  Species 
is  not,  for  the  most  part,  of  a  personal  character  except  in  the 
case  of  the  struggles  of  the  males  of  polygamous  animals. 
The  ordinary  struggle  for  existence  among  them  is  merely 
that  of  food-seeking,  where  all  possess  the  same  organs  and 
habits  but  in  varying  degrees  of  excellence.    Those  which  have 
the  greater  strength,  hardiness,  or  intelligence  are  more  likely 
to  get  their  portion  of  food  at  the  expense  of  the  weaker,  and 
thus  to  propagate  their  qualities.    Sometimes  however  animals 
live  directly  at  the  expense  of  their  own  species.     Young 
spiders  before  escaping  from  the  cocoon  in  which  they  are 
hatched  devour  each  other,  thus  instituting  an  acute  phase  of 
the  struggle  for  existence  iri  the  place  of  the  protection  pre- 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION. 

pared  by  parental  care.  Many  fishes  are  known  to  devour 
their  own  young.  We  have  all  had  occasion  to  wonder  what 
becomes  of  the  small  frogs  in  a  box  containing  large  ones. 
The  struggle  between  the  males  for  the  possession  of  the 
females  has  resulted  in  the  development  of  many  interesting 
adaptations.  The  struggle  may  take  the  form  of  actual  com- 
bat in  connection  with  which  organs  of  offense  and  defense 
are  found.  Such  are  the  horns,  tusks,  spurs,  manes,  and  even 
the  excessive  size  of  the  males  as  compared  with  the  females. 
Manifestly  the  same  qualities  which  make  a  male  a  formidable 
rival  to  another  are  likely  to  be  of  service  to  himself,  his  mates, 
and  his  young,  and  thus  to  the  species,  in  protecting  them  from 
the  attack  of  their  enemies  among  other  species.  The  com- 
petition between  males  is  not  all  of  this  stressful  kind  however. 
It  is  believed  by  many  naturalists  that,  in  those  instances  where 
simple  mating  rules,  those  males  with  the  most  striking  colors, 
pleasant  voices,  and  winning  ways  displace  their  less  favored 
rivals  and  thus  tend  to  accumulate  by  natural  (sexual)  selec- 
tion the  adaptations  of  this  class. 

154.  The  individuals  of  one  species  of  animals  may  often 
be  practically  indifferent  to  the  presence  of  those  of  other 
species.  Their  relation  is  simply  that  of  competing  for  the 
general  food  supply  and  thus  assisting  in  the  elimination  of  the 
unfit  in  all  species.  They  may  graze  in  the  same  pasture,  swim 
in  the  same  pool,  or  even  be  parasitic  on  the  same  host,  and 
have  no  other  relation.  From  this  as  the  simplest  relationship 
we  may  pass  by  gradual  stages  to  the  most  intimate  friendships 
and  the  most  bitter  antagonism.  Every  species  is  indifferent 
to  some  and  hostile  to  other  of  the  species  which  surround  it; 
and  man  is  no  exception  to  the  rule.  It  is  a  perversion  of 
manifest  fact  to  pretend  that  all  animals  are  of  some  use  to 
man. 

I55-  We. have  seen  that  the  individuals  of  a  given  species 
are  engaged  in  a  struggle  among  themselves  for  the  means  of 
subsistence,  and  that  in  certain  cases  they  form  communities 
or  colonies — a  kind  of  organic  corporation — in  order  to  meet 


n6 


ZOOLOGY. 


more  successfully  the  demands  made  upon  them  by  their  en- 
vironment. Similar  partnerships  may  be  formed  by  animals 
of  different  species.  The  simplest  of  these  associations  are 
known  as  commensalism  or  "  mess-mateism"  in  which  the 
degree  of  dependence  and  mutual  advantage  is  perhaps  not 
very  great.  As  instances  may  be  cited  the  occupancy  of  the 
same  burrows  by  the  prairie  dog  and  a  species  of  owl;  the 
attachment  of  barnacles  to  whales  and  sharks ;  the  hundreds  of 
species  of  other  Insects  which  live  in  the  nests  of  ants;  the 
lodging  of  fishes  and  other  animals  in  the  body-cavity  of  some 
of  the  large  tropical  sea-anemones  or  among  the  tentacles  of 
some  of  the  Hydrozoa.  Each  member  of  the  association  can 
live  without  the  other,  but  for  some  reason  they  often  occur 
together.  The  way  in  which  species  of  rats  and  mice  follow 
man  and  occupy  his  habitations  perhaps  may  be  considered 
under  this  head. 

156.  Symbiosis.1 — Under  this  term  are  included  even  closer 
relationships  between  members  of  different  species,  where  there 

FIG.  55. 


FIG.   55.     Hermit  crab  in  the  shell  of  a  Gasteropod.     After  Morse. 

Questions  on  the  figure. — What  structural  adaptations  has  the  hermit- 
crab  to  this  mode  of  life?  What  conceivable  gain  has  such  a  habit?  What 
animals  are  cited  as  symbiotic  with  the  hermit-crab? 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION. 


seems  to  be  a  distinct  advantage  accruing  to  both  members  of 
the  partnership  sufficient  to  account  for  it.  The  relation 
of  the  ants  to  the  aphides  or  plant-lice  which  they  capture  may 
be  so  described.  The  aphides,  although  captives,  are  nour- 
ished, often  at  great  expense  of  labor  to  the  ant,  on  the  food 
which  they  most  prefer,  and  in  return  the  ants  use  the  sweet 
secretions  of  their  bodies  as  food.  Certain  hermit-crabs, 

FIG.  56. 


FIG.  56.     Argynnis   cybele   on   thistle.     Natural   size.     Photo   by    Folsom. 

Questions. on  the  figure. — For  what  purpose  does  the  butter-fly  visit 
the  thistle?  What  special  adaptations  does  the  butter-fly  possess  for  this 
mode  of  life?  What  is  the  gain  to  the  thistle  from  the  visits? 

whose  habit  it  is  to  occupy  gasteropod  shells  as  a  home  into 
which  they  insert  the  soft  posterior  part  of  the  body,  cultivate 
friendly  relations  with  a  sea-anemone  which  becomes  attached 
to  the  shell,  often  with  the  active  help  of  the  crab.  In  this 
case  the  anemone  is  supposed  to  conceal  the  hermit  and  to 
help  protect  it  by  means  of  its  nettling  cells,  and  in  return  is 


Il8  ZOOLOGY. 

carried  about  to  fresh  fields,  and  enjoys  a  portion  of  the  food 
broken  up  by  the  strong  pincers  of  the  crab.  Observers  claim 
that  the  crab  offers  choice  morsels  of  food  to  its  companion. 
When  the  crab  by  reason  of  its  growth  needs  a  new  home  it  is 
said  to  transplant  the  anemone  thereto.  These  must  be  looked 
upon  as  very  remarkable  adaptive  instincts.  Symbiosis  is 
probably  more  common  between  animals  and  plants  than 
among  animals.  The  most  interesting  of  these  latter  are  seen 
in  the  so-called  "  ant-loving  "  plants,  in  which  the  plant  pro- 
duces special  homes  or  special  foods  for  the  ants,  and  the  ants 
in  return  protect  the  plant  from  the  ravages  of  other  leaf- 
cutting  ants  or  hurtful  insects.  Certain  sea-anemones  possess 
unicellular  algae  imbedded  in  the  cells  of  the  entoderm.  These 
algae  derive  their  nourishment  from  the  wastes  of  the  animal 
tissues  and  supply  oxygen  and  possibly  other  matter  to  the 
cells  in  which  they  lie.  The  close  relation  between  the  struc- 
ture and  instincts  of  insects,  on  the  one  hand,  and  the  form  of 
flowers,  their  products  and  needs,  on  the  other,  illustrates  a 
symbiotic  adaptation  which  has  long  attracted  students  both 
of  botany  and  zoology.  See  Fig.  56. 

157.  Library  Studies. — Make  a  report  concerning  the  various  myrme- 
cophilous    plants.     Accumulate    all    the    supposed    instances  •  of    symbiosis 
which  your  library  records.     How  do  the  lichens  illustrate  symbiosis? 

158.  The  Preying  Habit.— The  effects  of  this  habit  are 
stamped  upon  the  structure  and  activities  of  both  the  pursuer 
and  the  pursued.     It  is  in  this  relation  that  nature  is  indeed 
"  red  in  tooth  and  claw."     While  in  general  the  same  organs 
and  habits  which  are  of  value  in  the  capture  of  prey  are  useful 
in  the  defense  of  the  possessor,  it  is  possible  to  find  a  series 
of  adaptations  of  an  offensive  character  and  others  more 
specially  of  defensive  value.     The  curved  claws  and  sharp 
teeth,  the  stealthy  approach,  the  sudden  spring,  and  the  great 
agility  of  the  one  are  met  by  the  timidity,  the  keen  senses, 
the  fleetness  of  the  other.     We  can  see  that  these  defensive 
adaptations  must  keep  pace  with  the  offensive  else  the  prey 
would  be  exterminated,  which  would  entail  no  less  surely  the 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION. 

destruction  of  their  enemies  than  if  these  should  lose  their 
power  of  capturing  their  prey. 

159.  Adaptations  for  Protection. — In  addition  to  the 
alternatives  of  fighting  or  fleeing,  the  animals  which  are  preyed 
upon  have  very  interesting  and  effective  qualities  that  make 
for  safety.  Many  forms,  as  the  Crustacea,  have  permanent 

FIG.  57. 


FIG.   57.     Nestling  Mourning  Doves    (Zenaidura  macrourd).     From  U.    S.    Dept.   Agri- 
culture Year-Bopk,    1900. 

Question  on  the  figure. — Is  there  anything  suggestive  of  protective 
markings?  What  are  the  nesting  habits  of  the  dove?  What  character  of 
nest  is  constructed? 

outer  coverings;  most  mollusks  have  a  box  arrangement  into 
which  they  can  retire  when  threatened  by  attack;  others  by 
burrowing  or  otherwise  come  to  occupy  obscure  corners  in 
nature  where  enemies  find  it  difficult  to  follow.  Forms  as 
widely  different  as  the  mole  and  the  chamois  find  safety  in 
retirement.  This  hiding-theme  may  be  wrought  out  in  ways 
almost  equally  effective  by  what  is  called  protective  resem- 
blance. By  this  is  meant  that  the  animal  becomes  less  easily 


1 2O  ZOOLOGY. 

distinguished  from  its  environment  because  of  its  color,  or 
form,  or  both.  This  resemblance  may  be  to  some  particular 
object,  or  merely  a  general  harmony  of  color  with  the  sur- 
roundings. As  illustrative  of  the  latter  head  we  may  cite  the 
quail  among  the  dead  leaves  and  grasses,  the  sober-hued  lizard 
on  the  logs,  the  green  caterpillars  or  tree-toads  among  the 
leaves;  the  tawny  color  of  desert  animals,  the  white  fur  of 


FIG.  58. 


FIG.   58.     A   sea-horse, — Phyllopteryx  eques.     From   Eckstein. 

Question  on  the  figure. — Compare  this  figure  of  sea-horse  with  figures 
of  other  species  and  note  the  chief  difference  between  them  and  the 
typical  fishes  in  external  characteristics.  What  about  the  figure  suggests 
protective  resemblance?  At  what  point  does  the  tail  of  the  fish  end? 

arctic  forms,  the  transparency  of  many  marine  animals.  In- 
deed the  great  majority  of  animals  show  some  traces  of  re- 
semblance to  the  surroundings,  since  it  is  alike  advantageous 
to  the  predaceous  and  to  their  prey.  In  some  instances  there  is 
the  ability  to  change  color  with  changing  environment,  as  in 
the  tree-toads,  the  chameleon,  and  in  some  fishes.  This  is  not 
by  the  direct  action  of  the  light  on  the  pigment  cells  but  by 
reflex  action  of  the  nervous  system  stimulated  through  the 
eyes. 

Many  other  animals  become  inconspicuous  by  reason  of  a 
resemblance  to  special  objects.  It  is  among  the  insects  that  the 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.         121 

most  numerous  illustrations  of  this  are  found.  The  walking- 
stick  insect  appears  as  dead  twigs  when  not  in  motion.  Many 
butterflies  resemble  leaves,  when  at  rest.  A  noted  instance  is 
Kallima  which  is  a  large  species,  conspicuous  when  flying  be- 

FIG.  59- 


FIG.   59.     Walking-stick  insect    (Diapheromera  veliei)   on. twig.     Natural   size.     By 

J.    W.    Folsom. 

Questions  on  the  figure. — To  what  group  of  insects  does  this  belong? 
Do  you  see  any  reason  to  suppose  that  it  illustrates  protective  resemblance? 

cause  of  blue  and  orange  patches  on  the  upper  surface  of  the 
wings.  The  wings  are  folded  when  at  rest  and  the  lower  sides 
are  colored  and  marked  so  like  a  dead  leaf  that  the  deception 
is  very  complete.  The  larvae  of  some  of  the  geometrid  moths, 
often  called  "  measuring-worms,"  are  remarkably  like  the 
twigs  on  which  they  crawl,  both  in  color  and  shape.  This  is 
made  more  striking  by  the  presence  of  roughnesses  on  the  sur- 
face which  suggest  buds,  and  by  the  possession  of  muscles 
which  enable  them  to  support  themselves  rigidly  outstretched 
for  hours  by  means  of  the  posterior  legs  alone,  so  that  the 
axis  of  the  body  makes  an  angle  with  the  branch. 

Other  instances  of  special  devices  whereby  animals  protect 
themselves  are  found  in  the  electric  organs  of  some  eels  and 
other  fishes,  in  the  poisonous  fluids  with  or  without  special 
stinging  organs,  as  in  ccelenterates,  bees,  some  spiders,  a  few 


;  •::.  :  :v 


n  the  repulsive  odors 

J~:;~      .i 
.:  ::^:e    :.:: 


ih  a  spedal  odor  or  color  by  which  its  enemies 
may  icmgaiae  the  fart,  h  is  not  likely  to  prove  of  any  great 

—  -  •-;--   ::    :::c   ^::.::-  L.  ?-:<>es?:r-^:    :   -     ::    :.   -  v.^  r   ::::--;   ::: 

• 

FCHT  sflDBar  rcascKis  annnais   with  ^"^F'Y  are 
:       '  ;  .     i   :     ;  _^   :  ~  :  :_  :  ;  ~  :~"  ;.~-.-  /.~f          :    ;-;   :..  ~f-    ~.~~.  ~  '  ^ 

of  warxra^s.    The  "  mooardx    one  oi  oar  large  con- 
ImUci  flies  is  aa  iHostfaticKt 

..  ~:"~      r    T.-fTr      1,  f        Af7-   A"    .   ! 

ciatioxi  of  color  wiih  the  possession 


ot  the   tiuerm    are  quite  Jifl« 


•.'••   •••• 

,  ._.     . 


- : ;  .: .'.  r  -.     :      • 


..„  - 


•       •     ,,  > 


.-,'.   •>. 


I24 


ZOOLOGY. 


or  on  the  body  of  the  host  where  they  undergo  partial  develop- 
ment as  parasites.  In  other  instances  the  parasite  must  spend 
its  whole  life  in  the  body  of  one  or  more  hosts.  These  are 
called  permanent  parasites. 

FIG.  61. 


FIG.  61.     Caterpillar   of  Platysamia   cecropia   parasitized.     From   Lugger. 

Questions  on  the  figure. — Seek  in  your  reference  literature  all  figures 
and  references  to  caterpillars  attacked  by  parasites.  Why  would  cater- 
pillars be  rather  favorable  hosts  for  parasites?  What  are  a  few  of  the 
parasitic  enemies  of  caterpillars  ?  What  economic  importance  has  this 
phenomenon  ? 

In'  addition  to  the  drain  on  the  resources  of  the  host,  the 
presence  of  the  parasite  may  so  irritate  the  tissues  of  the  host 
as  to  produce  abnormal  growth  and  disease  therein.  In  many 
of  the  transient  parasites  the  life  of  the  individual  host  is  of 
no  consequence  after  the  end  of  the  period  of  parasitism  and 
hence  the  entire  destruction  of  the  host's  body  may  occur  just 
as  truly  as  in  the  ordinary  preying  species.  Very  profound 
modifications  occur  in  the  structure  of  the  parasite,  which  are 
the  outcome  of,  and  in  part  an  adaptation  to,  the  special  mode 
of  life.  There  is  usually  a  degeneration  of  the  organs  of 
digestion,  of  motion,  and  of  sensation,  since  the  parasite  de- 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.         125 

pends  on  the  host  for  the  performance  of  these  functions.  The 
explanation  of  this  degeneration  of  useless  or  unused  organs 
is  not  quite  certain.  It  is  known  that  disuse  causes  structures 
to  deteriorate  in  the  life  of  the  individual,  and  some  naturalists 
claim  that  part  of  this  loss  is  transmitted  to  the  next  genera- 
tion. The  claim  is  denied  by  many,  who  are  disposed  to  con- 
sider that  it  is  merely  a  case  of  natural  selection  working  for 
simplification  of  organs  and  the  economizing  of  materials. 
The  reproductive  organs  on  the  contrary  become  much  more 
complicated  and  the  reproductive  elements  are  produced  in  great 
abundance.  This  is  an  adaptation  to  the  difficulties  involved 
in  finding  the  special  host  in  which  development  may  proceed. 
This  is  more  striking  because  many  parasites  require  two  dif- 
ferent hosts  in  order  to  complete  the  life  cycle,  and  great 
mortality  accompanies  the  passage  from  one  host  to  another. 
A  good  illustration  of  such  parasites  is  the  tape-worm  which 
infests  the  trout  in  Yellowstone  Lake.  The  larvae  enter  the 
tissues  of  the  trout  and  by  their  ravages  weaken  and  kill  the 
host.  The  dead  fish  are  eaten  by  pelicans.  The  worms  de- 
velop to  the  adult,  sexual  condition  in  the  digestive  canal  of 
this  second  host  and  the  eggs  or  young  embryos  escape  into 
the  water  with  the  excreta  and  from  there  are  taken  up  by 
other  trout  whose  destruction  is  again  wrought  by  the  tissue- 
infesting  larvae.  This  passage  from  one  host  to  another  prob- 
ably arose  and  is  helped  by  the  carnivorous  habit  among  ani- 
mals. 

The  parasites  are  almost  exclusively  invertebrates.  The 
worms  and  arthropods  furnish  the  most  numerous  representa- 
tives. The  gregarines,  among  the  Protozoa,  are  internal  para- 
sites, sometimes  being  parasitic  zvithin  the  cells.  There  are  only 
a  few  parasitic  vertebrates,  and  these  are  transient.  They  be- 
long to  the  lower  fishes  (lamprey,  Fig.  62). 

Parasitism  is  a  very  successful  adaptation  to  a  much  limited 
environment  in  which  the  organism  has  bartered  its  original 
powers  for  a  life  of  comparative  ease.  The  only  necessity 
still  resting  upon  it  is  in  the  matter  of  reproduction,  and  the 


126 


ZOOLOGY. 
FIG.  62. 


FIG.  62.     Lake    Lamprey    {Petromyson   marinus  unicolor)    clinging  to    Sucker.      (From 
Bull.   U.   S.  Fish  Commission,  by  Surface.) 

Questions  on  the  figure. — Does  it  seem  that  this  is  an  instance  of 
parasitism  or  simple* preying?  What  special  organs  has  the  lamprey  adapt- 
ing it  to  this  habit?  What  references  can  you  find  to  the  breeding  habits 
of  the  lamprey? 

success  with  which  this  needful  function  is  accomplished  shows 
us  that  the  parasite  must  be  considered  well  adapted  to  its 
conditions,  notwithstanding  its  degeneracy.  Its  chief  hazards 
are  met  in  the  passage  from  host  to  host  and  these  are  over- 
come by  the  carnivorous  and  omnivorous  habits  of  hosts  and 
the  extraordinary  powers  of  multiplication  on  the  part  of  the 
parasites. 

161*.  Practical  Exercises. — Enumerate  all  the  parasites,  transient  and 
permanent,  known  to  infest  man,  and  find  to  what  groups  of  animals  they 
belong.  Report  on  the  habits  of  the  principal  parasites  on  man :  as  tape- 
worm, trichina,  etc.  What  other  hosts  are  demanded  to  complete  the  life 
cycle?  What  are  the  principal  sanitary  conclusions  to  be  reached?  Ex- 
amine the  mouth-parts  of  the  mosquito  (see  Fig.  63).  To  what  kind  of 
feeding  are  they  adapted? 

162.  Habits  and  Instincts  in  Relation  to  Adaptation.; — 

In  the  study  of  adaptations  there  is  constant  danger  lest  we 
come  to  consider  that  structures  alone  are  adaptive.  In  reality, 
adaptation  in  the  manner  of  doing  things  is  quite  as  important 
as  in  the  structure  of  the  organs  by  which  work  is  done. 
When  even  the  simplest  organisms  are  acted  on  by  an  external 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.          127 

stimulus  they  respond  to  it  in  some  way.  This  response  may 
be  either  advantageous  or  disadvantageous  to  the  organism. 
If  unfavorable,  the  result  may  be  disastrous.  If  favorable 
later  repetitions  of  the  stimulus  are  all  the  more  likely  to  be 
answered  by  the  same  kinds  of  response  as  in  the  first  instance. 

FIG.  63. 


m.  p. 


FIG.  63.  The  head  of  female  Mosquito  (Culex).  After  Dimmock.  a,  antennae;  c, 
clypeus;  h,  hypopharynx;  m,  mandibles;  ma.,  maxillas;  m.p.,  maxillary  palpus;  /, 
labium;  la.,  labrum  (epi-pharynx). 

Questions  on  the  figure. — In  what  way  and  for  what  purpose  are  the 
mouth-parts  of  the  mosquito  used?  What  are  the  probable  functions  of 
the  antennae?  Compare  the  antennae  and  the  mouth-parts  of  a  male  and 
female  mosquito.  See  also  Fig.  40.  Mention  some  respects  in  which  the 
mosquito  is  adapted  to  its  mode  of  life?  What  extent  of  horopter  do 
its  eyes  command?  To  what  degree  is  the  mosquito  parasitic? 

This  individual  acquirement  of  a  special  mode  of  responding 
to  stimuli  is  known  as  habit.  Since  responses  in  higher  organ- 
isms occur  by  means  of  the  nervous  system  we  rightly  asso- 
ciate habits  with  the  nervous  activities.  In  reality,  however, 
mere  protoplasm  may  acquire  these  habitual  modes  of  action 
and  one  might  say  that  all  such  adaptations  are  dependent  on 
the  power  of  protoplasm  to  respond  to  external  stimuli.  By 
reason  of  this  power  of  adaptive  responses,  organisms  may 
become  habituated  or  acclimatized  to  changes  in  their  environ- 
ment, their  habits  or  responses  changing  according  to  the 
necessities  of  the  case.  It  is  a  matter  of  common  observation 
that  animals  can  thus  gradually  be  brought  to  the  endurance 
of  conditions  which  would  originally  have  killed  them.  Such 
must  have  been  true  of  the  animals  which  have  come  to  live 


1 28  ZOOLOGY. 

in  the  waters  of  hot  springs.  Such  must  have  been  the  way 
in  which  other  animals  were  changed  from  the  marine  to  the 
fresh-water  habit,  since  all  fresh-water  animals  are  believed  to 
have  been  derived  from  marine  forms. 

Similarly,  in  the  history  of  any  species  those  individuals 
which  respond  in  suitable  or  advantageous  ways  to  the  stimuli 
brought  to  bear  on  them  are  selected  from  generation  to  gen- 
eration in  preference  to  those  not  so  responding,  and  in  the 
course  of  time  certain  modes  of  action  become  characteristic 
of  the  species,  even  without  the  necessity  of  individual  ex- 
perience. In  other  words  the  protoplasm  has  become  so  modi- 
fied in  a  series  of  generations  that  responses  of  a  definite  kind 
may  be  expected  of  it,  which  cannot  be  looked  upon  as  in- 
dividually acquired  habits.  These  are  instincts  and  embrace 
many  of  the  most  interesting  activities  which  have  been  men- 
tioned as  characteristic  of  animals.  The  instincts  of  feeding, 
mating,  and  the  like  are  examples.  If  instincts  are  in  conflict, 
the  stronger  prevails.  In  this  possibility  of  situations  arising 
in  which  the  instincts  are  in  conflict,  or  are  unequal  to  a  correct 
solution,  lies  the  advantage  of  intelligence  and  choice  as  adap- 
tations whereby  correct  responses  may  be  made  to  external 
conditions.  Of  the  utmost  importance  in  the  development  of 
intelligence  is  the  introduction  of  imitation,  of  training,  of 
experience,  of  memory, — factors  more  or  less  represented  in 
the  activities  of  all  the  higher  animals.  It  is  necessary  to  re- 
member that  what  we  call  intelligence  does  not  arise  suddenly 
in  the  animal  kingdom  and  is  not  confined  to  the  highest  ani- 
mals. Many  of  the  acts  usually  spoken  of  as  instinctive  are 
not  purely  so,  but  are  the  results,  in  part,  of  imitation,  parental 
or  social  training,  and  individual  experience,  and  are  therefore 
to  be  classed  as  intelligent. 

163.  The  Dispersal  of  Animals  and  the  Formation  of 
Special  Faunas. — In  section  136  we  see  that  every  point  occu- 
pied by  the  individuals  of  any  species  becomes,  under  natural 
influences,  a  centre  of  distribution  from  which  the  species  will 


INDIVIDUAL   DIFFERENTIATION    AND    ADAPTATION.         1 29 

spread  in  all  directions,  unless  kept  back  by  adequate  barriers. 
Thus  we  should  expect  all  animals  to  be  found  all  over  the 
earth  if  all  the  conditions  were  equally  suitable  and  all  animals 
were  equally  adaptable  to  varying  conditions.  This  however 
is  not  so.  Species  have  unequal  powers  of  adaptation  to  the 
different  conditions  and  thus  it  comes  tq  be  that  certain  groups 
of  species  adapted  to  some  special  environment  will  be  found 
together  in  certain  regions,  but  will  be  absent  from  others. 
The  total  animal  life  of  any  region  is  known  as  its  fauna. 

164.  The   Original   Home   of   Animals,   and   the    Sea- 
faunas. — There  can  be  no  reasonable  doubt  that  animal  life 
began  in  the  sea  and  close  to  its  surface,  and  probably  not  close 
to  the  shore.    From  this  region  the  various  nooks  and  crannies 
of  the  earth  have  been  occupied,  until  now  it  seems  that  there 
is  no  place  which  does  not  have  animals  suited  to  its  conditions. 
The  fauna  of  the  surface  of  the  mid-ocean  is  known  as  the 
pelagic  fauna.    It  is  made  up  largely  of  Protozoa ;  certain  more 
or  less  transparent  types  of  invertebrates,  as  worms,  jelly- 
fishes,    tunicates;   many   minute   Crustacea   and   fishes.     The 
abyssal  or  deep-sea  fauna  contains  representatives  of  all  types 
of  animals  from  protozoa  to  fishes,  notwithstanding  the  dark- 
ness and  the  great  pressure  of  the  water.    Many  of  the  forms 
are  highly  modified,  differing  markedly  from  the  correspond- 
ing species  found  in  other  life-regions.    The  littoral  or  shore 
fauna  is  the  most  varied,  abundant,  and  interesting  of  the 
sea- faunas.     Indeed*  there  is  no  place  on  the  earth  where  life 
is  more  abundant.    This  is  true  because  of  the  wonderful  food 
supply  broken  up  by  the  waves,  and  the  great  variety  of  phys- 
ical conditions  at  the  meeting  of  land  and  water. 

165.  Library  Exercises.— What  are  the  special  conditions,  of  each  of 
the  regions  indicated  in  the  preceding  section,  which  are  likely  to  be  favor- 
able or  unfavorable  to  life?     Illustrate  more  fully  the  typical  forms  char- 
acterizing each  region?    Find  instances  of  the  special  adaptations  which 
seem  peculiarly  advantageous  to  some  of  the  animals   frequenting  each 
region. 

1 66.  Fresh-water  Faunas. — From  the  littoral  regions  of 
the  sea,  animals  doubtless  originally  migrated  into  the  brackish 

10 


I3O  ZOOLOGY. 

water  of  the  mouths  of  rivers.  Thus  certain  types  came  to 
inhabit  the  fresh  waters  of  the  streams,  and  as  the  result  of 
the  adaptations  thus  made  necessary  new  species  arose  dis- 
tinctly different  from  their  relatives  which  remained  in  the 
sea.  The  most  of  the  branches  or  phyla  of  the  animal  king- 
dom have  their  fresh-water  representatives,  but  very  few 
species  of  the  sponges,  the  jelly-fish  group,  and  none  of  the 
star-fish  group  have  left  the  salt  water.  Some  species  of 
animals,  as  the  salmon  and  eels,  pass  back  and  forth  from  fresh 
to  salt  water  in  obedience  to  their  spawning  or  other  instincts, 
but  these  are  not  very  numerous. 

167.  From  the  fresh- water  fauna  or  from  the  ocean  shore 
fauna  have  come  those  species  which  have  acquired  the  power 
of  breathing  by  means  of  the  air.  These  embrace  some  worms 
and  mollusks,  the  insects,  and  the  vertebrates  above  the  fishes. 
This  adaptation,  which  is  one  of  the  most  important  acquired 
in  the  history ^of  animal  life  on  the  earth,  may  have  come  about 
by  the  gradual  or  periodic  drying  up  of  fresh  water  basins, 
or  by  means  of  temporary  excursions  to  the  land,  such  as  we 
see  some  water  forms"  capable  of  enduring  today.  Several 
types  of  these  terrestrial  animals  have  achieved  a  more  or  less 
complete  mastery  over  the  air  (aerial  fauna)  by  means  of 
flight.  Chief  among  these  are  the  insects,  the  first  group  to 
accomplish  the  task;  a  group  of  reptiles  in  early  geological 
times;  the  birds;  and  a  few  mammals  (as  the  bats).  Animals 
after  passing  from  one  region  to  another  may  in  their  descend- 
ants reoccupy  their  old  habitat.  Thus  the  whales  and  seals  are 
air  breathing  mammals  and  are  probably  descended  from  land 
forms,  but  have  become  aquatic.  The  same  is  true  of  some 
reptiles.  Many  birds  have  lost  their  powers  of  flight  and  have 
become  purely  terrestrial. 

Other  divisions  of  the  continental  and  oceanic  faunas  into 
geographical  faunas  are  made,  depending  on  the  climatic  con- 
ditions and  the  geological  history  of  the  regions.  The  prin- 
ciples governing  this  division  are  too  complicated  for  our  pres- 
ent purposes. 


INDIVIDUAL    DIFFERENTIATION    AND    ADAPTATION.         131 

1 68.  Summary. 

1.  It  is  necessary  to  consider  the  individual  not  merely  as 
a  group  of  cells  and  tissues  but  as  a  unit  acting  and  being  acted 
upon  by  all  external  forces  and  by  other  organisms. 

2.  Characteristics   derived   from   the  parents   through   the 
union  of  the  sexual  cells,  or  by  the  non-sexual  modes  of  repro- 
duction, are  described  as  hereditary.    Other  parental  influences 
have  nothing  to  do  with  heredity.     Heredity  may  preserve  old 
qualities  or  give  rise  to  new  ones. 

3.  Individuals  vary  as  the  result  (i)  of  internal  conditions 
and  changes,  the  causes  of  which  are  obscure,  and  (2)  of  dif- 
ferences in  the  environment.     The  environment  may  produce 
very  important  changes  during  the  single  life  of  the  individual. 

4.  The  food  supply  of  animals  is  limited,  since  all  ultimately 
depend  on  plants;  any  species  multiplying  at  its  average  rate 
of  increase  could  in  a  short  time,  if  unchecked,  stock  the  earth 
up  to  its  limits  of  support ;  that  this  does  not  occur  is  due  to  a 
struggle  for  food  among  the  excessive  numbers  which  are 
born,  whereby  only  a  small  percentage  of  them  reach  maturity. 
In  the  main,  those  survive  which  possess  some  qualities  which 
tend  to  fit  them  for  the  environment  in  which  they  find  them- 
selves.    These  are  thus  enabled  to  transmit  their  qualities  to 
their  offspring,  the  fittest  of  which  are  again  chosen.     The 
result   is   adaptation,   and   the  process   is  known  as   natural 
selection. 

5.  A  similar  result  is  effected  by  man  in  domestic  animals 
by  artificially  selecting  individuals  in  accordance  with  the  pos- 
session of  certain  features.    The  resulting  forms  are  frequently 
very  unsuited  to  the  natural  environment,  and  could  not  sur- 
vive if  left  to  themselves. 

6.  As  the  result  of  various  causes  animals  become  dispersed 
from  their  point  of  origin,  and  in  becoming  adapted  to  the 
different  regions  into  which  they  go,  or  through  variation 
within  a  given  region,  give  rise  to  new  varieties.     When,  by 
any  means,  these  groups  have  become  perfectly  adapted  to 
their  new  special  environment  and  permanently  different  from 


13 2  ZOOLOGY. 

their  parent  stock  and  from  each  other,  without  intermediate 
individuals  which  manifestly  connect  the  varieties,  they  are 
recognized  as  new  species.  Through  the  influence  of  heredity 
and  by  natural  selection  these  differences  may  accumulate, 
apparently  to  any  amount. 

7.  The  nutritive   function  relates  particularly  to  the  con- 
tinued existence  of  the  individual;  the  reproductive  function 
looks  to  the  continuance  of  the  species,  and  is  a  tax  on  the  in- 
dividual.    Nature  has  specially  favored  those  organisms  in 
which  an  increasing  degree  of  energy  is  given  to  the  produc- 
tion of  the  young.    As  it  is  sometimes  expressed,  nature  sacri- 
fices the  individual  to  the  welfare  of  the  species. 

8.  Animals  become  adapted  to  all  the  influences  that  tend  to 
make  or  mar  their  success  in  life.     The  more  powerful  the 
influence  the  more  certain  the  adaptation,  because  the  destruc- 
tion is  the  more  certain  in  case  of  failure.     The  principal 
classes  of  adaptations  are, — those  relating  to  the  using  of  the 
favorable  and  resisting  the  unfavorable  features  of  the  inani- 
mate environment;  those  assisting  in  the  obtaining  of  food 
whether  vegetable  or  animal;  those  of  mating  and  care  of 
young;  those  of  offense  and  defense,  in  predaceous  animals 
and  their  prey.     The  relations  and  adaptations  range  all  the 
way  from  indifference  to  friendship,  and  from  feeding  at  the 
same  table  on  the  one  hand,  to  the  utmost  antagonism  on  the 
other. 

9.  Perhaps  the  most  important  and  the  least  understood  of 
the  series  of  adaptations  which  animals  acquire  are  those  con- 
nected with  the  nervous  system  and  its  functions : — the  habits, 
instincts,  and  intelligence  of  animals.     They  are  inseparable 
from  those  already  enumerated,  and  yet  in  fundamental  im- 
portance they  form  a  group  of  their  own.     They  seem  pri- 
marily to  depend  upon  the  irritability  of  protoplasm  which 
enables  it  not  merely  to  respond  but  to  become  permanently 
changed  by  that  response — a  kind  of  organic  memory.    From 
this  fact  acclimatization  and  adjustment  become  possible. 

10.  In  being  scattered  from  their  starting  place,  animals 


INDIVIDUAL   DIFFERENTIATION    AND    ADAPTATION.         133 

with  similar  powers  of  response  and  adaptation  come  to  be 
located  in  the  same  kinds  of  conditions.  This  results  in  faunas 
more  or  less  characteristic  of  all  the  important  kinds  of  en- 
vironments :  as  marine,  brackish  water,  fresh  water,  terrestrial, 
aerial,  cavern  faunas,  etc. 

11.  The  origin  of  animal  life  was  in  the  ocean,  and  from 
these  marine  types  it  is  believed  that  all  other  forms  of  animal 
life  have  come,  by  gradual  adaptation  to  their  present  mode 
of  life. 

12.  The  various  climatic  zones  of  the  earth  and  the  principal 
geographical  regions  are  characterized  by  distinct  forms  of 
life.    For  example,  the  lake  life  of  Africa  differs  from  that  of 
North  America,  and  similarly  for  all  the  various  types  of 
fauna.    An  analysis  of  such  facts  and  an  explanation  of  them 
belongs  to  the  geographical  distribution  of  animals. 

169.  Topics  for  investigation,  in  field,  laboratory  and  library: 

1.  What  constitutes  individuality  in  animals? 

2.  In  what  respects    (enumerate)    and  to  what  degree  have  you  ever 
noticed  variety  in  a  given  species?     In  the  offspring  of  a  pair  of  parents? 

3.  Have  you  ever  observed  any  changes  in  structure  in  animals  which 
could  reasonably  be  attributed  to  change  in  environment?    Give  evidence. 

4.  Does  use  or  disuse  produce  changes  in  the  organs  of  an  individual? 
Why?     Give  illustrations. 

5.  Enumerate   some   facts   of   your   own   observation    which    illustrate 
heredity. 

6.  Cite  observed  instances  of  associations  among  animals  of  the  same 
species,  and  determine  as  well  as  you  can  from  your  observations  what 
ends  are  gained  by  the  association. 

7.  Make  an  effort  to  classify  a  series  of  objects,  noting  carefully  your 
basis  of  classification;  that  is,  the  characters  which  you  select  in  separat- 
ing and  grouping  the  individuals.    The  teacher  can  make  this  a  most  in- 
structive exercise.     A  few  objects  of  considerable  diversity  may  be  chosen, 
as  sand,  pebbles,  shells,  crystals,  a  plant,  an  animal,  and  the  student  may 
be  required  to  examine  each  as  fully  as  he  can,  write  out  the  characters 
which  he  discovers  as  belonging  to  each,  being  sure  that  he  uses  a  simple 
and  observed  feature  in  each  statement.     On  the  basis  of  these  recorded 
observations  let  him  compare  and  group  the  objects.     Or  take   a  large 
number  of  relatively  similar  individuals  and,  without  stopping  to  write 
their  characters,  let  the  student  place  or  distribute  them  in  groups  near 
or  remote  from  each  other  in  proportion  to  their  unlikenesses,  allowing 
intermediate  forms  to  stand  between.    Afterward  he  may  be  caused  to 
determine  and  justify  his  classification  and  to  see  whether  other  classifi- 


134  ZOOLOGY. 

cation  could  be  made  with  a  different  basis.  Gasteropod  shells,  illustrating 
varieties  of  the  same  and  different  species;  beetles,  butterflies;  grass- 
hoppers; or  even  books  of  diverse  shape,  binding  and  contents  may  be 
used. 

8.  Can  you  suggest  any  cause  for  the  degeneracy  of  parasites? 

9.  Cite  instances  from  your  own  observation  in  which  animals  use  the 
leap  or  spring  in  capturing  prey  or  escaping  enemies.     Why  is  it  a  pecu- 
liarly advantageous  adaptation? 

10.  Cite  instances  of  the  food-storing  instinct,  with  all  observed  details. 
What  is  the  most  remarkable  fact  about  them?     How  is  it  useful? 

11.  From  reading  and  observation  would  you  say  that  there  is   any 
definite  relation  between  the  instinct  for  home-building  and  parental  care? 

12.  Study  sleep  among  animals.     What  is  its  relation  to  rest?    Is  it 
found   in   the   lower  animals?     To  what  is   sleep   an   adaptation?     When 
does  sleep  commonly  occur  among  animals?     Why?     Do  plants  show  any 
sleeping  qualities? 

13.  What  are  the  principal  geographical   faunas   recognized  by  zoolo- 
gists.    Enumerate  the  more  important  means  by  which  dispersal  of  ani- 
mals from  one  region  to  another  occurs.     What  are  the  chief  barriers  to 
the  dispersal  of  land  animals?  of  aquatic  animals? 

14.  Study  the  different  authors  to  which  you  have  access,  as  to  the 
significance  of  the  terms  species  and  variety  (or  sub-species). 

15.  What  were  the  older  views  concerning  the  "  fixity  "  or  invariability 
of  species? 

16.  What  are  the  different  views  of  the  "  Origin  of  Species,"  as  based 
on  the  views  of  the  meaning  of  species? 

17.  What  is  the  essential  difference  between  the  theories  of  "natural 
selection  "  and  "  definite  variation  "  as  explaining  adaptation  of  organisms 
to  their  environment? 

18.  What  additional  ideas  are  introduced  by  the   "mutation"   theory 
of  De  Vries? 


CHAPTER    IX. 
A   GENERAL   PREVIEW   OF   THE   ANIMAL   KINGDOM. 

Before  undertaking  the  study  of  the  special  groups  into 
which  animals  are  arranged  because  of  their  apparent  kinships, 
it  will  be  advantageous  for  the  student  to  look  briefly  at  the 
whole  field  of  animals, — the  "  animal  kingdom."  See  Fig.  64. 

170.  Mammals. — Beginning  with  man  himself  it  is  easy  to 
see  that  there  are  numerous  animals  (as  the  apes  and  monkeys ; 
the  various  quadrupeds,  as  the  horse,  ox,  dog,  cat,  bears  and 
squirrels;  the  whales  and  seals;  and  many  others)  which  differ 
much  in  general  appearance  from  him  but  are  like  him  in  very 
many  remarkable  particulars.     For  example,  they  all  bring 
forth  their  young  alive  and  in  a  more  mature  condition  than 
is  usual  for  other  types  of  animals,  the  young  being  carried 
in  a  special  organ  of  the  mother's  body,  often  until  develop- 
ment is  well  advanced.    After  birth  the  mother  produces  milk 
in  special  glands  for  the  nourishment  of  the  young  to  a  still 
more  mature  stage.    This  is  seen  in  no  other  group  of  animals 
beside  the  mammals.     The  skin  produces  hair  or  wool  as  a 
covering  for  the  body.    Man  differs  from  the  other  mammals 
in  certain  particulars  but  not  nearly  so  much  as  he  and  they 
differ  from  other  animals. 

171.  Birds. — Another  well-developed  and  numerous  group 
of  animals  is  the  class  known  as  birds.    There  is  scarcely  an- 
other class  of  animals  so  easy  to  distinguish  at  sight  as  this. 
They  equal  or  surpass  the  mammals  in  specialization,  but  are 
very  different  from  them.    They  are  especially  to  be  recognized 
by  the  body-covering  of  feathers,  the  modification  of  the  front 
limbs  into  wings  for  purposes  of  flight,  and  the  fact  that  the 
jaws  are  sheathed  in  horny  matter  and,  at  least  in  present  day 
birds,  do  not  possess  teeth. 

135 


i36 


Mammals 


ZOOLOGY. 
FlG.  64. 

'Birds 

10000 


Reptiles 

3000 


mphibians 


Mollusks 
Tunicata,  etc.      (     25000 

v 


Coelenterates 

r 

Protozoa 

4000 

FIG.  64.     Diagram  showing  the  general   relations  of  the  chief  divisions  of  the  animal 
kingdom.     The  number  of  species  belonging  to  each  is  roughly  approximate,  only. 

172.  Reptiles. — This  is  a  class  recognized  by  zoologists 
which  is  not  nearly  so  easy  to  define  or  to  identify  as  either  of 
the  preceding.  This  is  partly  because  the  animals  composing 
it  differ  more  among  themselves  than  in  the  other  classes.  It 


GENERAL    PREVIEW    OF    THE    ANIMAL    KINGDOM.  137 

includes  snakes,  lizards,  turtles,  and  crocodiles.  The  reptiles 
have  some  features  which  indicate  that  they  may  be  distantly 
related  to  both  birds  and  mammals,  as  well  as  to  th$  next 
class.  This  is  an  additional  reason  why  the  group  of  reptiles 
is  a  difficult  one  to  define.  In  general  they  may  be  recognized 
by  the  fact  that  their  bodies  are  covered  by  scales  or  plates 
instead  of  hair  or  feathers.  They  always  breathe  oxygen  from 
the  air,  as  do  birds  and  mammals.  They  usually  have  only 
three  chambers  to  the  heart  whereas  in  the  former  groups 
there  are  four.  The  blood  is  not  constantly  warm  as  in  birds 
and  mammals.  They  lay  eggs  very  much  like  those  of  birds. 

173.  Amphibians. — In  external  appearance  the  members  of 
this  class  often  look  somewhat  like  reptiles,  and  they  have 
certain  possessions  in  common  with  them,  as  the  cold  blood 
and  the  3-chambered  heart.     They  are  especially  noteworthy 
from  the  fact  that  they  begin  life  breathing  oxygen  from  the 
water  as  fishes  do,  and  later  in  life  lose  their  gills,  acquire 
lungs,  and  get  their  oxygen  from  the  air,  as  do  the  reptiles 
and  higher  forms.     Amphibians  include  the  frogs,  toads  and 
salamanders.     This  is  not  a  very  important  group  in  nature, 
but  is  intensely  interesting  to  the  student  of  zoology  because 
it  seems  to  be  a  connecting  link  between  the  air-breathing  and 
the  water-breathing  forms. 

174.  Fishes. — Fishes   are   characterized  by  the   fact  that 
they  breathe  by  means  of  gills  throughout  life.    The  body  is 
often  scaly;  the  appendages  are  fin-like;  the  blood  is  cold,  and 
the  heart  has  two  chambers.     They  are  beautifully  adapted 
to  life  in  the  water  and  are  easily  recognized. 

175.  Vertebrates  and  Invertebrates. — All  the  animals  of 
which  we  have  thus  far  spoken  agree  in  certain  particulars. 
They  all  possess  a  dorsal  rod  of  supporting  matter  (notochord; 
see  §  341),  which  is  often  surrounded  by  cartilage  or  bone  (the 
vertebral  column).     The  nervous  system  in  all  of  them  is 
dorsal  to  this  rod  and  to  the  digestive  tract,  and  is  tubular  in 
character.     The  heart  is  ventral  to  the  digestive  tract  and  the 


138  ZOOLOGY. 

blood  has  red  corpuscles.  They  are  called  Chordata  or  Verte- 
brata.  All  other  animals,  with  the  exception  of  a  few  which 
seem  intermediate  in  some  respects,  are  classed  as  Inverte- 
brates, and  agree  in  general  in  the  following  facts : — there  is 
no  notochord  or  vertebral  column;  the  nervous  system  is 
chiefly  ventral  to  the  digestive  tract;  the  heart,  when  present 
is  dorsal;  and  the  blood  usually  has  only  colorless  cells.  The 
principal  phyla  of  the  Invertebrates  follow. 

176.  Arthropoda. — This  is  the  most  numerous  phylum  of 
the  animal  kingdom.     It  embraces  crayfish,   lobsters,   crabs 
(Crustacea),  which  for  the  most  part  have  gills  and  live  in 
water;  the  Insects,  as  bees,  flies,  beetles,  butterflies,  etc.,  which 
usually  live  in  the  air  and  get  their  oxygen  from  it ;  the  spiders, 
whose  habits  and  appearance  are  somewhat  similar  to  those 
of  the  insects.    Arthropods  are  especially  to  be  recognized  by 
the  fact  that  their  bodies  are  segmented,  are  bilaterally  sym- 
metrical, and  have  paired  jointed  appendages  to  many  of  the 
segments.     In  addition  to  this  there  is  a  covering  of  resistant 
substance  (chitin)  developed  by  the  skin.     This  serves  for  the 
protection  of  the  animal  and  for  the  attachment  of  the  muscles 
within. 

177.  Mollusca. — This  phylum  of  invertebrates  includes  the 
snail,  clam  and  oyster,  the  squid  and  devil-fish,  and  their  kind. 
They  differ  very  much  among  themselves  but  agree  in  the  lack 
of  segmentation  of  their  bodies,  in  the  absence  of  paired  ap- 
pendages,— and  in  those  types  most  commonly  known  to  the 
student,  in  the  presence  of  a  shell  of  one  or  two  valves,  which 
is  secreted  by  a  fold  of  the  skin  called  the  mantle.     While 
many  of  the  mollusks  are  lowly  in  organization  and  in  intelli- 
gence, one  group  of  them — that  which  includes  the  squid, — 
has  the  most  highly  developed  brain  found  below  the  verte- 
brates.    It  occupies  among  the  invertebrates  somewhat  the 
place  which  man  has  among  the  mammals. 

178.  Echinoderms. — These  are  easily  recognized  by  the 
possession  of  five  or  more  arms  or  rays  in  the  adult  stage. 


GENERAL    PREVIEW    OF    THE    ANIMAL    KINGDOM.  139 

Usually  a  skeleton  is  developed  in  the  skin.  This  is  often  cov- 
ered with  spines,  and  from  this  fact  the  phylum  has  its  name. 
They  are  marine  and  are  poor  movers, — a  few  being  fixed  by 
stalks  to  objects  in  the  ocean.  The  starfish,  sea-urchin  and 
sea-lilies  are  representatives. 

179.  Annulata    (Segmented   Worms). — This  phylum   is 
similar  to  the  arthropods  in  that  the  body  is  bilaterally  sym- 
metrical, is  segmented,  and  has  paired  appendages  to  many  of 
the  segments.    It  differs  from  them  in  the  fact  that  the  append- 
ages, when  present,  are  not  jointed  but  are  merely  setae  or  hairs 
in  sockets  or  on  fleshy  prominences.     The  segments  are  more 
nearly  homonomous  than  in  typical  Arthropods.     The  earth- 
worm, many  types  of  aquatic  worms,  and  leeches  are  included 
here. 

1 80.  Unsegmented    Worms     (embracing    numerous    ill- 
assorted  animals  of  doubtful  relationship).     Here  may  be  in- 
cluded a  number  of  small  groups  many  of  which  have  long 
been  grouped  with  the  Annulata  and  called  "  worms."     They 
are  not  sufficiently  alike  to  be  regarded  as  one  distinct  phylum. 
The  majority  of  them  are  bilaterally  symmetrical,  unsegmented 
and  without  appendages.     They  differ  from  the  Mollusks  in 
that  they  do  not  possess  a  mantle  and  do  not  secrete  a  shell. 
Many  of  them  are  parasitic.    Among  these  animals  of  doubtful 
relationship    may    be    included    the  "  flat-worms,"    "  round- 
worms,"  the  nemertea,  rotifers,  and  others. 

181.  Coelomata  and  Coelenterata. — All  the  animals  thus 
far  considered  possess  during  some  stage  of  life  a  more  or  less 
developed  body  cavity  or  ccelom  (see  §  56)  distinct  from  the 
digestive  tract.     For  this  Veason  they  are  sometimes  known 
collectively  as  Ccelomata.    All  the  remaining  many-celled  ani- 
mals have  a  general  cavity  which  serves  both  as  a  body  cavity 
and  a  digestive  tract  (gastro-vascular  cavity), — or  to  speak 
more  exactly,  there  is  no  true  body  cavity.     Of  these  the 
phylum  Coelenterata  are  the  chief  illustration.     Here  belong 
the  jelly-fish,  sea-anemone,  corals.     They  are  all  aquatic  and 


140  ZOOLOGY. 

are  more  or  less  tubular,  sac-shaped  animals  often  attached  by 
one  end,  with  the  mouth,  which  also  functions  as  the  anus,  at 
the  other  surrounded  by  clusters  of  tentacles.  Many  secrete 
skeletons,  and  some  form  immense  attached  colonies. 

182.  Porifera. — This  group,  to  which  belong  the  sponges, 
is  sometimes  classed  with  the  Coelenterata.     While  similar  to 
them  in  habit  the  sponges  are  much  less  highly  organized  and 
unified.    Instead  of  a  single  mouth  opening  into  the  digestive 
tract,  sponges  have  many  openings  or  pores  (whence  the  name 
Porifera)  which  are  the  beginnings  of  tubes  entering  a  central 
cloaca  or  sewer.     This  is  in  reality  not  a  true  digestive  tract. 
It  communicates  with  the  exterior  by  one  or  more  large  pas- 
sages.   They  are  attached  and  usually  form  large  colonies  by 
budding. 

183.  Protozoa. — All  the  preceding  phyla  of  animals  con- 
sist, in  the  adult  stage,  of  many  cells  among  which  there  is 
more  or  less  differentiation.     In  all  of  them  the  adult  passes 
through  stages  in  which  the  cells  are  arranged  in  at  least  two 
layers  (ectoderm  and  entoderm;  see  §  53),  from  which  the 
tissue-masses  arise.     These  animals  are  known  as  Metazoa. 
In  the   remaining  phylum — the  Protozoa — the  animals   are 
single  cells,  or  at  most  loose  aggregations  of  similar  cells. 
They  are  the  lowest  of  animals  and  are  for  the  most  part  in- 
visible to  the  naked  eye. 

184.  An  Artificial  Key  to  the  Phyla  of  the  Animal  Kingdom. 

Many-celled   animals    METAZOA. 

With  true  ccelom   Coelomata. 

Possessing  notochord   (and  often  vertebral  column), 

Phylum  Chordata. 
Possess  functional  gills. 

Throughout  life   Class  Fishes. 

In  embryonic  life  only  (with  a  few  exceptions), 

Class  Amphibia. 
Do  not  possess  functional  gills. 

Epidermal  covering  of  scales   Class  Reptiles. 

Epidermal  covering  of  feathers    Class  Birds. 

Epidermal  covering  of  hair    Class  Mammals. 


GENERAL    PREVIEW    OF    THE   ANIMAL    KINGDOM.  14! 

Without    notochord    Invertebrata. 

Bilaterally  symmetrical   (chiefly). 
Body  made  up  of  segments. 

Paired  appendages  jointed   Phylum  Arthropoda. 

Paired  appendages  unjointed   Phylum  Annulata. 

Body  unsegmented ;  without  paired  appendages. 
With  mantle — often  secreting  shell, 

Phylum  Mollusca. 

No  mantle   Unsegmented  Worms. 

Radially  symmetrical  in  adult    Phylum  Echinodermata. 

Without  true  ccelom. 

With  a  single  mouth,  which  also  functions  as  an  anus :   stinging 

cells    Phylum   Ccelenterata. 

With  numerous  incurrent  openings   or  pores,   and   only  one — or 

few — excurrent.     No  stinging  cells Phylum  Porifera. 

Single-celled  animals    (chiefly)    Phylum  Protozoa. 


CHAPTER   X. 
PHYLUM    I— PROTOZOA. 

LABORATORY  EXERCISES. 

Without  compound  microscopes  this  branch  of  animals  can- 
not be  studied  with  profit  in  the  laboratory.  The  Amoeba  is 
one  of  the  most  interesting  of  the  Protozoa  and  serves  well  to 
illustrate  the  simplest  forms  of  animal  life,  but  large  specimens 
in  sufficient  numbers  for  profitable  study  in  an  elementary  class 
are  usually  so  difficult  to  secure  at  the  right  time  that  it  be- 
comes a  question  whether  the  teacher  should  be  advised  to 
depend  on  them.  My  advice  is,  make  every  arrangement  you 
can  to  secure  them,  use  them  for  demonstration  or  study 
whenever  they  appear,  but  depend  on  Paramecium.  Perhaps 
the  surest  method  for  securing  Amoeba  is  to  chop  up  the  soft 
parts  of  three  or  four  fresh-water  mussels,  placing  the  pieces, 
together  with  the  shells,  in  a  large  shallow  basin.  Allow  a 
gentle  stream  of  water  to  drip  into  this.  This  keeps  the  water 
slightly  agitated,  causes  it  to  run  over,  and  prevents  an  undue 
accumulation  of  bacteria.  The  addition  of  a  little  of  the  sur- 
face mud  secured  from  the  bottom  of  several  streams  or  ponds 
will  make  the  success  of  the  preparation  all  the  surer. 
Amcebas  should  appear  at  the  surface  of  the  mud,  about  the 
shells,  or  at  the  margins  of  the  vessel  near  the  surface  of  the 
water.  Test  all  these  places  every  day,  and  sooner  or  later  the 
Amcebas  are  practically  sure  to  be  found.  Paramecia  will  be 
likely  to  occur  in  the  same  preparation.  Any  abundant  Proto- 
zoan which  may  appear  may  be  studied  instead  of  Paramecium 
or  in  addition  to  it,  by  means  of  the  outline  below.  The  mode 
of  securing  the  materials  should  be  explained  to  the  class  to 
make  clearer  the  habits  of  these  organisms.  * 

185.  Paramecium. — This    Protozoan    may    be    obtained 
readily  by  allowing  fresh-water  Algse,  with  hay  or  leaves,  to 

142 


PROTOZOA.  143 

decay  in  water.  This  infusion  should  be  examined  every  day. 
If  the  bacteria  become  too  abundant  some  of  the  surface  water 
may  be  poured  off  and  fresh  water  added.  The  paramecia, 
which  are  just  visible  to  the  naked  eye,  appear  as  a  whitish 
cloud  in  the  water  or  may  accumulate  as  a  film  at  the  surface. 
Often  a  sufficient  number  for  study  may  be  secured  by  scraping 
with  a  scalpel  the  matter  which  accumulates  on  the  sides  of  the 
vessel  just  beneath  the  water  surface,  even  when  they  are  not 
sufficiently  numerous  to  cloud  the  infusion.  The  cover-glass 
should  be  supported  by  sediment  or  by  bits  of  cover-glass. 
Make  outline  sketches  of  everything  which  can  be  thus  shown. 
I.  With  the  low  power  of  the  microscope  study  the  follow- 
ing points : 

1.  Activities. — Describe,  and  figure  as  well  as  possible,  the 
nature  of  all  the  movements  of  which  the  animal  seems  capable, 
using  arrows  to  indicate  directions.     Can  you  distinguish  an 
anterior  from  a  posterior  end?   By  what  characteristics? 

Do  you  find  any  reasons  for  believing  that  the  Paramecia 
are  sensitive  to  external  influences?  What  evidences?  To 
what  sorts  of  influences  do  they  respond?  Do  they  avoid  ob- 
jects? Do  they  collide  with  each  other  in  motion?  Do  they 
tend  to  collect?  Where?  Are  they  as  active  at  the  end  of  the 
hour  as  at  the  beginning  ? 

Make  a  new  preparation  in  which  the  Paramecia  are  uni- 
formly distributed  in  a  drop  of  water.  Place  a  small  grain  of 
salt  at  the  edge  of  the  drop.  What  is  the  result  ?  Watch  the 
individuals  under  the  microscope  as  they  come  into  the  salt 
solution.  On  a  new  preparation,  try  similarly  a  minute  amount 
of  acetic  acid  (YO  to  l/2  per  cent,  solution)  applied  with  a 
capillary  tube.  Compare  results.  Try  sugar,  quinine. 

Do  you  discover  any  instances  of  division  or  conjugation? 
If  so,  describe. 

2.  General  form  of  the  body.     How  would  you  describe 
its  shape?  To  what  degree  is  it  capable  of  change?  Is  the  body 
symmetrical?   Give  evidences.    Make  diagrams  showing  your 
idea  of  a  cross-section  through  the  middle;  also  of  one,  one- 
third  way  from  each  end. 


144  ZOOLOGY. 

II.  With  the  high  power,  study, — 

3.  Cilia:  where  found?    Are  they  uniform  in  length?     How  do  they 
act?    What  results  do  they  produce?     (Place  a  small  amount  of  water 
containing  finely  powdered  indigo  or  carmine  at  edge  of  cover-glass.     If 
the  movements  are  too  rapid  a  little  gelatine  added  to  the  water  will  be 
of  advantage.) 

4.  Find  the  month,  with  the  oral  groove  leading  to  it.     Position  and 
shape?     How  are  food  particles  captured?     Can  you  find  them  within  the 
body  (food  vacuoles}  t    Do  the  food  vacuoles  move  within  the  cell?     If 
so,  trace  their  course?    What  finally  becomes  of  them?    Evidences? 

5.  Contractile  vacuoles  (clear  spherical  objects  rhythmically  disappear- 
ing  and    reappearing).     Number?     Position?    Rate    of    contraction?     Do 
they  contract  at  the   same   time?     What  becomes  of  the  clear  material 
during   the    contraction    of   the   vacuole?    Are    they    deep    or   superficial 
structures?    Your   evidences?     Does    change    of    temperature    cause    any 
change  in  their  rate  of  contraction? 

6.  Distinguish  between  the  inner  mass  of  protoplasm   (endosarc*)   and 
an  outer  layer   (ectosarc).    What  are  the  characteristics  of  each  as  re- 
gards motion,  clearness,  firmness,  etc.?     Note  the  changes  in  these  por- 
tions on  the  addition  of  dilute  acetic  acid  or  iodine  at  the  edge  of  the 
cover-glass. 

7.  Discover  if  possible  nuclear  bodies.    These  are  not  usually  recog- 
nizable without  careful  staining.     Place  at  the  edge  of  the  cover-glass,  in 
a  fresh  preparation  of  Paramecia,  a  5-10%   aqueous  solution  of  methyl 
green.     Compare  the  result  with  a  permanent  mount  stained  by  suitable 
methods  (see  "Suggestions  to  Teachers"). 

186.  Other  Protozoa. — If  the  class  is  supplied  with  microscopes,  the 
pupils  should  be  allowed  to  examine  stagnant  water  for  as  many  types 
of  protozoa  as  may  be  found.  Allow  them  to  compare  these,  noting  the 
points  of  similarity  and  difference  in  general  structure  and  activities. 
Especially  profitable  protozoa  for  laboratory  work  are  the  green  flagellate 
infusorian,  Euglena,  which  often  tinges  the  water,  or  forms  a  green  scum 
over  shallow  pools  of  water;  the  colonial  ciliate  form,  Vorticella,  found 
attached  to  submerged  objects  in  ponds  or  pools  of  slowly  moving  streams 
in  which  there  is  considerable  decaying  organic  matter.  The  colonies  are 
easily  visible  to  the  naked  eye.  Stentor  is  a  very  large  trumpet-shaped 
infusorian  which  may  be  alternately  attached  and  free-swimming.  It  lives 
upon  submerged  sticks  and  leaves  and  may  often  be  found  attached  to 
the  sides  of  vessels  in  which  such  matter  has  been  placed.  In  all  such 
studies  and  identification  of  the  protozoa  the  question  of  evidence  of  the 
unicellular  character  of  the  organism  should  be  kept  before  the  student. 

DESCRIPTIVE  TEXT. 

187.  In  this  first  and  lowest  group  of  animals,  the  individ- 
uals of  which  consist  of  single  cells  or  loosely  associated  simi- 


PROTOZOA.  145 

lar  cells,  we  find  something  of  the  variety  of  shape  which  we 
observed  in  the  tissue  cells  of  the  higher  animals  (Chapter  V). 
The  Protozoa  are  especially  interesting  to  the  biologist  because 
they  represent  the  simplest  forms  of  animal  life  now  found 
on  the  earth  and  because  some  of  their  representatives  are  very 
like  some  of  the  simplest  plants.  Indeed  some  of  them  are 
claimed  by  both  the  botanists  and  the  zoologists.  It  also  seems 
probable  that  the  first  animal  life  to  appear  on  the  globe  had 
the  general  characteristics  of  some  of  the  Protozoa.  Whether 
some  type  of  protozoan  is  to  be  considered  as  the  ancestor  of 
the  higher  many-celled  animals  or  not,  it  is  true  that  we  find 
illustrated  here  in  the  simplest  possible  way  the  beginning  of 
all  those  functions  which  are  so  completely  distributed  among 
the  special  organs  of  the  complex  animals.  The  Paramecium 
does  in  a  simple  yet  satisfactory  way  all  that  any  living  animal 
needs  to  do  in  order  to  live  and  perpetuate  its  species. 


FIG.  65.     Amoeba,     ec.,    ectosarc;    en.,   endosarc,    containing   food  vacuoles    (f ) ;   n, 
nucleus;   p,   pseudopodium;   p.v.,    pulsating  vacuole. 

Questions  on  the  figure. — Define  the  various  terms  used  above  in 
describing  the  parts  of  the  amoeba.  What  changes  may  the  amoeba  undergo 
in  its  life  history?  Compare  figures  i  and  6. 

1 88.  General  Characters. 

1.  Mostly  unicellular  throughout  life.     May  have  one  or 
more  nuclei  (Figs.  66-69). 

2.  The  protoplasm  usually  consists  of  a  clearer  outer  por- 
tion (ectosarc)  and  a  more  granular  inside  portion  (endosarc) 
(Fig.  66,  ec,  en). 

ii 


146  ZOOLOGY. 

3.  There  is  usually  what  is  known  as  a  pulsating  vacuole, 
in  which  some  of  the  more  fluid  cell-contents  collects,  to  be 
forced  out  of  the  vacuole  again  by  the  contraction  of  the  denser 

protoplasm  (Fig.  66,  pv). 

FIG.  66. 


FIG.  66.  Paramecium  in  optical  section  (semi-diagrammatic).  A,  anterior  end;  c, 
cilia;  e.c.,  ectosarc;  e.n.,  endosarc;  f.v.,  food  "vacuole";  g,  gullet;  N,  meganucleus; 
n,  micronucleus;  o,  oral  groove,  leading  to  the  mouth;  p.v.,  pulsating  vacuoles  in  dif- 
ferent stages  of  contraction;  tr.,  trichocysts;  v,  food  vacuole  in  process  of  formation. 

Questions  on  the  figure. — In  what  sense  is  the  term  "  vacuole  "  descrip- 
tive of  the  structures  to  which  it  is  applied  in  Paramecium?  Describe  the 
special  adaptations  of  the  anterior  end.  Judging  from  their  distribution 
have  the  cilia  any  other  function  than  locomotion?  In  what  way  are  the 
food  vacuoles  formed?  Why  do  some  food  vacuoles  appear  lighter  than 
others  ? 

4.  Reproduction  is  effected  chiefly  by  dividing  into  two  or 
more  parts  or  cells,  which  occasionally  remain  associated.  The 
nucleus,  when  present,  divides  with  the  division  of  the  cell 
(Fig.  6). 

189.  Habitat. — Protozoa  in  their  active  stages  require 
abundant  moisture,  hence  they  are  found  in  water,  fresh  or 
salt,  and  as  parasites  in  the  bodies  of  other  animals.  The 
Sporozoa  are  parasitic.  Some  amoeboid  Rhizopods  infest  the 
digestive  tract  of  man  and  other  animals,  producing  irritation 
and  disease.  The  Infusoria  occur  in  water  in  which  there  is 
decaying  organic  matter  and  minute  organisms  of  various 
kinds.  Volvox  and  Euglena,  green  forms  often  classed  as 
Protozoa,  have  the  power  which  green  plants  possess  of  living 
on  the  inorganic  substances  found  in  ordinary  water. 


PROTOZOA.  147 

190.  Organization. — We  "cannot  say  that  Protozoa  have 
organs  in  the  sense  in  which  we  have  denned  that  term  hitherto, 
yet  they  are  certainly  organized.     The  organization  shows 
itself  in  the  nucleus,  in  the  distinction  of  ectosarc  and  endosarc, 
in  the  pulsating  and  food  vacuoles,  in  temporary  projections 
of  protoplasm  called  pseudopodia,  in  more  permanent  vibratile 
projections  of  the  ectosarc  known  as  cilia  or  flagella,  in  the 
mouth — found  in  many  forms,  in  cell-wall  and  secreted  skele- 
ton, in  delicate  contractile  fibres  in  the  ectosarc,  and  in  stalks 
for  attachment  to  objects  (see  Figs.  66  and  68).     By  means 
of  these  differentiations  all  the  functions  necessary  to  life  are 
performed.     There  are  many  colonial   Protozoa.      In   such 
(as  Volvox)  there  may  be  some  division  of  labor  among  the 
cells, — as  between  reproductive  cells  and  body  cells    (Figs. 
70,  70- 

191.  Nutrition. — The  parasites  absorb   food,  already  di- 
gested and  fitted  for  absorption,  directly  from  their  hosts. 
Most  of  the  free  forms  take  solid  particles  directly  into  the 
endosarc  through  permanent  or  temporary  openings  in  the 
ectosarc.     In  some  shelled  forms,  in  which  there  is  no  mouth, 
the  food  is  digested  outside  the  body  proper   (Fig.   72)   by 
the  pseudopodia.    These  envelop  the  food  and  gradually  trans- 
fer it  to  the  main  body  of  protoplasm.    In  the  other  instances 
the  digestion  takes  place  in  the  body  of  the  protoplasm.    The 
ferments  found  in  the  protoplasm  are  doubtless  responsible 
for  the  digestive  changes  and  act  in  much  the  same  way  as  the 
special  ferments  secreted  from  the  cells  of  the  digestive  glands 
in  the  higher  animals.     Circulation  is  effected  by  the  general 
protoplasmic  motion.    Respiration,  whereby  the  protoplasm  gets 
rid  of  CO2  and  receives  O,  occurs  through  the  cell  surface 
without  special  structures.     All  projections  of  the  cell-body 
assist  in  this  exchange  by  increasing  the  area  of  the  surface. 
Excretion  may  take  place  from  the  surface  of  the  cell,  and  it 
seems  probable  that  the  contractile  vacuole  has  an  excretory 
function. 


ZOOLOGY. 

192.  Movement. — The  majority  of  Protozoa  move  freely 
in  their  medium.     In  Amoeba  it  is  of  a  gliding  character  and 
is  effected  by  putting  forth  processes  into  which  the  protoplasm 
streams.     The  process  or  pseudopodium  thus  enlarges  at  the 
expense  of  the  body  of  the  cell  and  progress  is  had  in  the  direc- 
tion of  the  growing  pseudopodium.    The  direction  of  motion 
is  changed  by  the  breaking  out  of  new  processes  in  a  new 
direction.     In  those  Protozoa  which  have  a  cell-wall  special 
devices  become  necessary  to  enable  the  animal  to  move.    Most 
of  the  free-swimming  forms  possess  cilia  or  flagella,  which 
act  as  oars  on  the  water  and  thus  propel  them.     In  Stentor, 
Spirostomum,  Vorticella,  etc.,  there  are  clearly  defined  strands 
of  contractile  material  developed  in  the  ectosarc  by  which  the 
shape  of  the  animal  may  be  strikingly  changed.     In  the  at- 
tached forms  these  strands  extend  from  the  body  proper  into 
the  stalk.     Vorticella  (Fig.  68)  by  this  device  may  change  its 
position  with  much  suddenness.     Attached  forms  are  able  to 
break  loose  from  their  moorings  and  become  free-swimming 
for  a  time.     Still  other  species  are  encased  in  shells  and  are 
practically   destitute   of   the  power   of   independent   motion. 
Even  the  most  active  types  may  assume  the  non-motile  or  rest- 
ing stage,  by  which  they  pass  uninjured  through  such  unfavor- 
able conditions  as  drouth,  cold,  and  the  like. 

193.  Sensation. — All  the  Protozoa  show  more  or  less  sensi- 
tiveness to  external  conditions.     They  may  be  caused  to  con- 
tract and  move  by  mechanical   stimuli   such   as  contact  or 
jarring,  by  chemically  active  substances  in  the  water,  by  light, 
by   changes   in   temperature,   and   the   like.      Vorticella   and 
Spirostomum  are  exceedingly  sensitive  to  contacts;  Amoeba 
avoids  the  light;  many  forms  seem  to  find  their  food  as  the 
result  of  the  chemical  differences  in  the  water  and  may  be 
seen  to  swarm  about  suitable  objects ;  the  contractile  vacuoles 
of  many  forms  contract  more  rapidly  in  warm  than  in  cold 
water ;  Paramecia  tend  to  collect  in  groups  at  the  edge  of  the 
cover  glass,   around  air-bubbles,   about  green   filaments,   or 


PROTOZOA. 


I49 


without  any  foreign  matter.  So  far  as  we  know,  these  simple 
responses  do  not  give  evidence  of  special  organs,  but  merely 
represent  a  diffused  protoplasmic  irritability  and  power  of 
responding  to  stimuli  (§§  19,  20). 

FIG.  67. 


FIG.  67.  Paramecium.  i,  transverse  fission;  2-5,  stages  in  conjugation.  Lettering 
as  in  Fig.  66.  The  meganucleus  gradually  disintegrates  during  the  process  and  the 
micronucleus  by  two  successive  divisions  forms  four  micronuclei.  Two  of  these  dis- 
integrate. One  of  the  remaining  micronuclei  (n3)  in  each  animal  passes  into  the 
other  Paramecium  and  unites  with  the  stationary  micronucleus  (n4),  thus  fertilizing  it. 
Later  a  new  meganucleus  is  formed  in  each  animal  by  the  division  of  this  body. 

Questions  on  the  figure. — What  structures  divide  in  the  fission  of 
Paramecium?  Which  do  not?  Which  is  permanently  represented  in  the 
cell  during  conjugation,  the  micro-  or  the  mega-nucleus?  Which  seems  to 
correspond  most  nearly  to  the  ordinary  nucleus  of  higher  forms?  What 
really  transpires  in  the  act  of  conjugating?  Compare  this  with  more 
elaborate  figures  in  reference  texts. 

194.  Reproduction. — In  the  Protozoa  we  discover  methods 
of  reproduction  which  are  to  be  looked  upon  as  suggestions  of 
methods  found  in  the  Metazoa.  Reproduction  among  the 


ZOOLOGY. 


FIG.  68. 


FIG. 


FIG.  68.  A,  Vorticella,  a  stalked  ciliate  Infusorian:  i,  contracted;  2,  extended.  /, 
food  "vacuoles";  g,  gullet;  m,  contractile  fibre  (muscular);  n,  nucleus;  o,  mouth,  sur- 
rounded by  ciliated  disc;  p.v.,  pulsating  vacuole;  s,  stalk.  B,  a  colonial  type  similar 
to  Vorticella. 

Questions  on  the  figures.— Compare  the  internal  structure  of  Vorticella 
with  that  of  Paramecium  (Fig.  66).  What  are  the  principal  differences? 
Likenesses?  How  is  a  colonial  type  (as  B)  formed?  How  are  new  colonies 
started?  In  what  way  does  the  animal  become  extended  after  contraction? 
Compare  living  animal. 

FIG.  69.  A,  Euglena  viridis,  a  flagellate  Infusorian.  i,  typical  swimming  condition; 
2,  somewhat  contracted;  3,  spherical  resting  condition;  4,  encysted  stage  in  which  fission 
has  taken  place,  c,  cyst;  f,  flagellum;  n,  nucleus;  o,  mouth;  p.v.,  pulsating  vacuole; 
sp,  pigment  spot. 

B,  Podophrya,  a  stalked  Infusorian  bearing  tentacles  (*)•  P,  Infusorian  captured 
for  food;  s,  stalk. 

Questions  on  the  figures. — How  does  multiplication  in  Euglena  differ 
from  that  of  Paramecium?  What  are  the  differences  in  the  method  of 
feeding  employed  in  Vorticella  and  in  Podophrya?  What  is  the  structure 
and  function  of  the  tentacles  in  the  latter? 


PROTOZOA.  151 

Protozoa  is,  primarily,  mere  fission  or  division  of  the  cell-sub- 
stance. In  some  instances  this  division  is  little  more  than  an 
irregular  breaking  up  or  fragmentation  of  the  protoplasm.  In 
others,  one  or  more  buds  may  arise  from  the  parent  cell.  A 
more  typical  method  is  by  the  equal  division  of  the  parent  into 
two  new  individuals.  In  still  other  instances,  especially  among 
the  Sporozoa,  there  is  the  formation  of  a  cyst,  within  which 
the  protoplasm  rearranges  itself  in  numerous  small  bits  which 
finally  break  from  the  cyst  as  new  individuals.  In  all  such 
cases  the  old  nuclear  material  is  distributed  among  the  daugh- 
ter individuals.  There  are  indications  that  the  process  of 
division  carried  on  for  a  long  time  without  cessation  results  in 
a  gradual  loss  of  the  vitality  of  the  stock.  There  are  two  ways 
in  which  this  untoward  result  is  overcome,  so  that  a  kind  of 
rejuvenation  occurs.  In  the  first  place,  a  thick  wall  may  be 
formed  and  a  period  of  rest  ensue  (encystment) .  Or  in  the 
second  place,  there  may  be  a  temporary  (Paramecium)  or 
permanent  (Volvox,  Vorticella)  union  of  two  or  more  in- 
dividuals. This  is  conjugation.  '  The  essential  thing  in  con- 
jugation seems  to  be  the  introduction  of  new  nuclear  matter 
into  the  cell.  The  conjugation-cells  (gametes^  may  be  alike 
(Paramecmm) ,  or  diverse  (Vorticella  or  Volvox).  Parame- 
cium  may  reproduce  for  many  generations  by  division,  and 
then  two  individuals  may  conjugate,  exchange  certain  nuclear 
elements,  and  separate, — beginning  once  more  their  process  of 
division.  There  is  here  no  sign  of  sexual  dimorphism.  In  the 
colonial  species  however,  as  Vorticella  and  Volvox,  there  is 
the  union  and  permanent  fusion  of  individuals  (cells),  dis- 
tinctly different  in  form  and  size,  to  produce  the  new  indi- 
vidual. This  is  much  like  the  dimorphism  found  in  the  sexual 
cells  in  the  Metazoa  or  many-celled  animals,  and  illustrates 
heterogamy  (see  §  98).  Consult  Figs.  6,  67,  71. 

195.  History. — The  existence  of  the  Protozoa  was  prac- 
tically unknown  until  the  compound  microscope  came  into  use. 
A  naturalist  of  Holland  first  discovered  the  Infusoria,  and 


152 


ZOOLOGY. 


thus  opened  up  one  of  the  most  interesting  departments  of 
zoology.  It  was  not  until  the  middle  of  the  nineteenth  century 
that  the  simple,  unicellular  structure  of  the  Protozoa  was 

FIG.  70. 


FIG.  70.     Eudorina.     A  colony  of   16  flagellate  cells  imbedded  in  a  gelatinous  matrix. 

FIG.  71. 


FIG.  71.  Eudorina.  The  development  of  reproductive  bodies  within  the  colony 
from  the  ordinary  vegetative  cells  (.v).  f,  a  mass  of  female  cells;  m,  a  mass  of  male 
or  motile  cells;  f,  a  single  female  cell  surrounded  by  male  cells  (m') ;  w,  the 
boundary  of  the  original  colony. 

Questions  on  figures  70  and  71. — What  suggests  that  this  is  a  colony 
rather  than  an  individual?  What  suggests  the  reverse?  Compare  accounts 
in  other  texts  to  test  your  conclusions.  What  degree  of  differentiation  is 
shown  among  the  cells? 


PROTOZOA. 


153 


really  understood.  Many  of  them  can  endure  drying,  be  blown 
about  in  the  spore  stage,  and  then  take  up  active  life  again 
on  the  return  of  water,  so  that  thereupon,  in  a  few  hours,  In- 
fusoria may  literally  swarm  where  none  seemed  to  be.  This 
is  responsible  for  the  long  life  of  the  old  belief  that  they  arose 
by  "  spontaneous  generation,"  that  is,  without  parents.  It  is 
only  in  recent  years  that  this  belief  has  been  finally  disproved. 
It  is  known  that  they  do  not  appear  in  water  that  has  been 
boiled  and  kept  free  from  exposure  to  the  air. 

FIG.  72. 


FIG.  72.  A  compound  Foraminiferan — Nodosaria.  a,  aperture  of  shell;  /,  food 
particles  captured  by  the  strands  of  protoplasm  outside  the  shell;  n,  nucleus;  sh,  shell. 
1-4,  the  successive  chambers  of  the  shell;  i,  being  the  oldest. 

Questions  on  the  figure. — Does  this  seem  a  colony  or  a  single  in- 
dividual? Why?  Why  is  digestion  possible  outside  the  capsule?  Compare 
this  with  figures  of  Protozoa  in  which  there  is  no  large  aperture  to  the 
shell. 

196.  Classification  of  Protozoa. — The  following  are  the  principal 
classes  of  protozoa. 

Class  I.  Rhizopoda  (root-footed}.— ^y^:  Amoeba.  The  Rhizopoda  are 
amoeboid  in  form  with  pseudopodia,  which  may  be  either  blunt  (Fig. 
65)  or  slender  (Fig.  72).  The  protoplasm  may  be  naked  (Amoeba) 
or  may  secrete  a  shell  either  calcareous  (Foraminifera)  or  siliceous 
(Radiolaria} .  In  the  shelled  forms  the  pseudopodia  pass  out  through 


r. 


ZOOLOGY. 


openings  in  the  skeleton   (Fig.  73).     Reproduction  is  usually  by  division, 
or  by  the  formation  of  many  spores.     Encystment  frequently  occurs. 

Class  II.  Infusoria  (in  infusions}. — Types:  Paramecium,  Stentor,  Vor- 
ticella.  Predominantly  active  protozoa,  usually  without  shell,  but  with 
distinct  cortical  portion  from  which  project  permanent  vibratile  threads 
of  protoplasm  (cilia,  flagella,  or  tentacles),  from  the  possession  of  which 
the  sub-classes  are  named.  There  is  usually  a  permanent  mouth.  The 
nucleus  is  always  present  and  assumes  a  great  variety  of  shapes.  The 
infusoria  are  typically  free-swimming,  but  many  are  capable  of  attach- 
ment by  a  contractile  stalk,  to  foreign  objects  (V  orticella) .  Reproduc- 
tion is  normally  by  equal  division,  but  budding  and  spore  formation  occur. 
Conjugation  is  common,  and  may  be  either  temporary  or  permanent. 

FIG.  73- 


P' 


FIG.  73.  Actinomma,  a  radiolarian  with  a  shell  and  no  mouth.  A,  whole  animal 
with  a  portion  of  two  spheres  of  shell  removed.  B,  section,  showing  relation  of  proto- 
plasm to  the  skeleton,  c.,  central  capsule;  n,  nucleus;  p,  protoplasm;  o,  openings 
through  which  the  pseudopodia  extend.  (From  Parker  and  Haswell.) 

Class  III.  Sporozoa  (spore  animals). — Protozoa  predominantly  passive 
in  habit,  parasitic,  with  no  pseudopodia,  and  no  cilia  in  the  adult.  Re- 
markable for  encysted  resting  stages  and  spore  formation.  Conjugation 
often  precedes  the  formation  of  the  cyst. 

197.  Place  in  Nature. — Protozoa  are  an  important  element 
in  the  food  of  many  aquatic  animals.  Despite  their  minute 
size,  their  immense  numbers  make  them  important.  Together 
with  bacteria  they  serve  to  save  for  the  organic  world  much 
decaying  material  which  no  other  animals  could  utilize.  Rhi- 
zopod  shells  dropping  to  the  bottom  of  the  ocean  form  the 
"  ooze," — the  chalk  of  later  geological  epochs.  Other  forms 
of  limestone  also  are  produced  by  the  accumulations  of  these 


PROTOZOA.  155 

calcareous  shells.  Similar  masses  of  the  siliceous  shells  occur 
in  various  parts  of  the  earth.  Some  of  the  Protozoa,  especially 
the  parasitic  Sporozoa  produce  diseases  in  man  and  other  ani- 
mals. Malaria  and  yellow  fever  in  man  are  caused  by  Sporo- 
zoa in  the  blood.  In  both  these  diseases,  species  of  mosquitoes 
are  apparently  the  cause  of  the  introduction  of  the  spores  into 
the  human  system.  Texas  fever,  one  of  the  most  dreaded  of 
the  diseases  of  cattle,  is  believed  to  be  communicated  through 
the  cattle  tick,  in  which  the  sporozoan  producing  the  disease 
undergoes  a  portion  of  its  life  history. 

Pieces  of  such  protozoa  as  Stentor  have  been  shown  to  be 
able  to  regenerate  a  whole  animal,  provided  a  portion  of  both 
nucleus  and  protoplasm  are  present,  but  not  otherwise.  This 
shows  that  each  is  necessary  to  the  activities  of  the  animal. 
Because  they  are  lowly  and  simple  animals,  we  must  not  con- 
sider that  they  are  either  unimportant  or  unsuccessful  in  the 
struggle  for  existence.  Their  wonderful  reproductive  power 
insures  that  they  hold  their  own  whenever  the  conditions  are 
at  all  favorable  for  them.  They  occur  in  practically  all  the 
waters  of  the  earth,  increasing  or  decreasing  as  their  food 
varies  in  abundance. 

198.  Supplementary  Studies  for  the  Library. 

1.  The  reactions  of  Protozoa  to  light;  to  chemical  substances;  to  heat; 
etc. 

2.  Their  power   of   resistance   to   heat ;    cold ;    drouth.    The   practical 
results  thereof. 

3.  The  economic  importance  of  Protozoa. 

4.  What  is  "plankton"?    What  is  the  importance  of  its  study? 

5.  Conjugation   in    Protozoa.     Compare   methods   of   reproduction   and 
conjugation  in  the  various  groups.     Follow  the  nuclear  changes  in  con- 
jugation of  Paramecium. 

6.  Why  should  Volvox  and  Euglena  be  considered  animals  rather  than 
plants  ? 

7.  Diseases  in  man  or  animals  believed  to  be  caused  by  the  sporozoa. 
The  role  of  the  mosquito  in  the  life  history  of  the  sporozoa  causing  ma- 
laria and  yellow  fever.    The  bearing  of  these  facts  upon  infection  and  the 
management  of  these  diseases. 

8.  Forms  of  the  Protozoa  of  different  classes  as  shown  by  the  illustra- 
tions in  the  larger  text-books. 

9.  The  varying  form  of  the  nucleus  in  different  species  of  Protozoa. 


CHAPTER   XL 

PHYLUM    II— PORIFERA. 

LABORATORY  EXERCISES. 

199.  Grantia. — This  is  a  marine  sponge  and  in  consequence 
the  majority  of  schools  will  be  compelled  to  depend  upon  alco- 
holic material.  Grantia  occurs  along  our  New  England  coast, 
and  is  found  attached  to  piles  or  to  stones  a  few  feet  below  the 
low-tide  mark.  If  the  school  is  near  the  coast  the  living  sponge 
should  be  studied  in  a  basin  of  sea- water. 

1.  General  Form. —  (Keep  in  a  watch-glass,  covered  with 
the  preserving  fluid.)    Make  careful  outline  sketches  of  every- 
thing discovered. 

Note, — the  basal  or  attached  portion;  the  column;  the  free 
end.  How  do  the  ends  differ?  Are  there  any  openings? 

Do  you  find  any  connection  between  individuals  (budding)  ? 
Are  these  individuals  of  equal  size? 

2.  Structure. — Split  the  body  longitudinally  with  a  sharp 
scalpel,  and  examine  with  hand  lens  or  a  low  power  of  the 
microscope. 

Study, — body  wall;  cloaca  (internal  cavity);  the  relation 
of  the  cloaca  to  the  osculum  (the  opening  at  the  unattached 
end). 

By  what  is  the  osculum  surrounded?  Notice  in  the  wall  of 
Jhe  cloaca  the  minute  openings  of  the  radiating  tubes.  Do 
they  communicate  with  the  exterior?  What  are  the  functions 
of  the  osculum  and  of  the  pores?  Evidences? 

3.  Make  thin  cross  sections  with  a  razor,  mount  under  cover-glass,  and 
examine  further  for  points  in  2.     Notice  the  spicules.     Is  there  any  regu- 
larity in  -their  arrangement?    What  differences   in  shape  and  size  have 
you  discovered  in  the  spicules  from  different  regions  of  the  body? 

4.  Place  a  bit  of  the  sponge  in  a  small  amount  of  a  5%  solution  of 
caustic  potash  and  boil.     Examine  under  high  power,  and  draw  the  dif- 
ferently shaped  spicules. 

156 


PORIFERA.  157 

5.  Place  a  bit  of  the  sponge  on  slide  and  allow  weak  acetic  or  hydro- 
chloric acid  to  pass  under  the  cover.     Note  and  interpret  results. 

200.  Comparison  Demonstrations. 

1.  Fresh-zuatcr  Sponge. — In  portions  of  the  country  where  the  streams 
are  clear,  swift,  and  with  rocky  bottoms,  a  fresh-water  sponge  may  often 
be  found  which  will  be  valuable  to  compare  with  Grantia  or  substitute  for 
it.     It  grows  attached  to  submerged  objects  and  is  commonly  of  a  dirty 
greenish  color,  though  this  may  vary.    This  sponge  is  firm  and  gritty  to 
the  touch,  and  may  be  either  compact  or  branched.     Use  the  general  out- 
line prepared  for  Grantia,  noting  the  points  of  contrast.     Is  there  any- 
thing like  the   osculum?  the  cloaca?     Gemmules   or  reproductive  bodies 
may  occur  imbedded  in  the  flesh,  especially  at  the  base. 

2.  The  Sponge  of  Commerce. — This  is  merely  the  skeleton  of  a  sponge 
from  which  all  the  cellular  part  has  been  removed.     Select  a  small  rounded 
specimen.     Do  you  find  any  signs  of  the  attached  end?  of  an  osculum? 
Split  the  sponge  with  scissors,  beginning  with  an  osculum.     Are  there  any 
canals  as  in  Grantia ?    If  so,  what  is  their  arrangement?     Examine  a  small 
portion  of  the  skeleton  under  the  microscope.    Test  as  before  (for  calcic 
carbonate)   with  dilute  acid.     Is  the  skeleton  elastic?     Why? 

DESCRIPTIVE  TEXT. 

20 1.  The   Protozoa  are  unicellular  animals,   or  at  most, 
masses  of  similar  cells  in  a  more  or  less  globular  form.    This 
condition  is  comparable  to  the  morula  stage  of  the  embryos  of 
higher  animals  (see  §  52).     In  all  the  other  groups  (Metazoa) 
the  cells  at  some  stage  in  development  are  in  _at  least  two 
layers,  an  inner,  and  an  outer  or  superficial  layer,  a  structural 
condition  which  we  have  seen  at  its  simplest  in  the  gastrula 
(see  §  53).     The  exact  position  of  the  Porifera  in  the  animal 
series  has  long  been  a  matter  of  debate,  but  the  great  majority 
of  zoologists  agree  that  they  stand  below  all  the  other  Meta- 
zoa, presenting  transitional   features  between  the   Protozoa 
and  Metazoa.    For  this  reason  they  are  especially  interesting. 
Some  authors  include  them  with  the  next  phylum — the  Ccelen- 
terata.    They  possess  two  cell-layers,  but  the  division  of  labor 
among  the  cells  is  not  so  decided  as  in  the  Ccelenterata,  and 
the  individual  cells  .are  very  much  more  independent  of  each 
other  in  consequence. 

202.  General  Characters. 

i.  Porifera  possess  a  system  of  internal  chambers  through 
which  the  water  flows.    The  water  enters  by  means  of  many 


ZOOLOGY. 


minute  pores  at  the  surface,  passes  along  radiating  tubes  (m- 
current  channels)  to  the  central  cavity  (cloaca)  and  escapes 
through  one  or  more  larger  openings  (oscula)  at  the  unat- 
tached end.  There  is  no  true  coelom  (see  §  56). 

2.  Parts  are  arranged  about  the  central  cavity  but  not  usu- 
ally in  a  symmetrical  fashion. 

3.  There  are  two  distinct  layers,  ectoderm  and  entoderm. 
These  are  separated  by  a  gelatinous  mass  in  which  are  in- 
cluded cells  of  different  kinds   (mesenchyma  or  mesoderm) 


FIG.  74- 


FIG.  74. 


Leucandra,  a  simple  type  of  sponge.      (From  Delage  and  Herouard;   "  Traite 
de  Zoologie   Concrete.") 


Questions  on  the  figure. — What  is  the  position  of  the  osculum?  Which 
is  the  attached  end?  How  many  individuals  are  represented  in  the  cut? 

FIG.  75.  Diagrams  to  illustrate  the  development  of  one  of  the  simpler  types  of 
sponge:  i,  the  egg;  2,  section  of  16-  to  32-celled  stage;  3,  section  of  later  stage,  a 
ciliated  larva  (blastula)  ;  4,  gastrula;  5,  section  through  older  larva  which  has  become 
attached  by  the  end  containing  the  blastopore.  New  openings  break  through  by  the 
coalescence  and  perforation  of  the  ectoderm  and  entoderm,  and  a  form  results  such 
as  is  figured  in  Fig.  76.  a,  archenteron;  bl.,  blastopore;  ec.,  ectoderm;  en.,  entoderm; 
mes.,  mesenchyma;  s,  segmentation  cavity. 

Questions  on  the  figures. — What  terms  would  be  applied  to  the  cleav- 
age and  gastrulation  in  this  sponge?  What  is  suggested  as  to  the  mode  of 
forming  mesoderm?  The  attachment  of  the  sponge  by  the  blastopore  end 
of  the  larva  necessitates  what  later  development  ?  See  Fig.  76. 


PORIFERA. 


not  in  a  true  layer.     In  the  cells  of  the  mesenchyma  spicules 
are  produced,  forming  the  supporting  skeleton  (Fig.  77,  C). 

4.  Non-sexual    reproduction    is    prevalent,    but    dimorphic 
sexual  cells  are  also  formed  in  the  mesenchyma.    The  sexually 
produced  larva  is  free-swimming;  the  adult  is  attached. 

5.  Mostly  marine;  wholly  aquatic. 

FIG.  76. 


Y.C.... 


FIG.  76.  Diagram  of  simple  type  of  sponge,  more  mature  than  in  Fig.  75.  c,  cloaca; 
ch,  chambers,  lined  with  flagellate  entoderm;  e.p.,  external  pores;  i.p.,  internal  pores; 
mes.,  mesenchyma;  o,  osculum;  r.c.,  radiating  canals.  Other  letters  as  in  Fig.  75.  In 
the  adult  sponge  the  canals  and  flagellate  chambers  become  much  more  complex  than 
figured  here. 

Questions  on  the  figure. — What  portions  of  the  animal  are  lined  with 
ectoderm  ?  With  entoderm  ?  What  two  main  types  of  entoderm  are  figured  ? 
What  is  the  actual  nature  of  the  mesoderm  in  sponges?  Is  there  a  coelom 
(a  cavity  bounded  by  mesoderm)  ?  What  mechanical  advantage  do  you 
see  in  the  fact  that  the  water  currents  enter  by  way  of  the  radial  canals 
and  find  their  exit  through  the  osculum,  rather  than  the  opposite  direction? 
Compare  with  figure  77. 

203.  General  Form. — The  simpler  sponges  are  cylindrical 
or  vase-shaped  sacs  with  an  opening  (the  osculum)  at  the  un- 
attached end.  From  the  central  cavity  (cloaca)  of  the  sac 
numerous  radial  passages  pierce  the  walls  (Fig.  76),  and 


i6o 


ZOOLOGY. 


terminate  directly  or  indirectly  in  pores  at  the  surface  (whence 
the  name — Porifera).  In  the  more  complicated  sponges  there 
is  such  power  of  budding  and  lateral  growth  that  there  is 
formed  a  dense  tuft  of  sponge  made  up  of  many  individuals 
in  organic  connection  with  each  other.  In  such  sponges  the 
simplicity  of  the  internal  structure  is  lost,  and  the  cloaca  may 
branch,  opening  to  the  exterior  by  a  number  of  oscula.  The 
radial  passages  which  penetrate  the  wall  become  much 
branched  and  enlarged  in  special  regions  until  the  mesen- 
chyma  becomes  honey-combed  with  the  passages  and  cham- 
bers. No  animals  are  more  profoundly  influenced  by  their 
environment,  in  the  matter  of  the  special  form  which  the 
individual  assumes,  than  the  sponges.  Individuals  which  de- 
velop in  active  currents  differ  much  in  bodily  shape  from 
members  of  the  same  species  which  grow  in  sheltered  places. 
In  all  instances  the  form  assumed  appears  to  be  correlated  to 
the  external  conditions. 

204.  The  Structure  of  the  Body. — In  the  typical  condition 
the  body  of  a  sponge  consists  of  ectoderm  and  entoderm, 
with  a  gelatinous  mass  between  them  in  which  are  imbedded 
cells  of  various  kinds  and  spicules  of  hard  material  forming 
a  skeleton  (Fig.  77,  C).  The  ectQderm  is  usually  of  flattened 
cells  and  covers  the  exterior.  It  lines  the  pores  and  the  outer 
ends  of  the  passages  by  which  the  water  passes  to  the  interior. 
The  entoderm  lines  the  cloaca  and  the  radial  tubes,  and  is 
especially  well  developed  in  the  pocket-like  enlargements  of 
these  tubes,  when  they  occur  (Figs.  76,  ch\  77,  en2).  In  these 
passages  the  entoderm  is  more  columnar  in  shape  and  is  sup- 
plied with  flagella,  by  the  action  of  which  currents  of  water  are 
kept  flowing  inward  to  the  central  cavity.  The  middle  mass  or 
mesenchyma,  which  lies  between  these  two  layers  and  makes 
up  the  principal  thickness  of  the  body,  consists x  of  numerous 
cells  of  various  kinds  in  a  gelatinous  intercellular  substance 
(Fig.  77,  mes).  Some  of  these  cells  are  amoeboid  or  migra- 
tory, others  resemble  the  cells  of  connective  tissue,  others  se- 


PORIFERA.  T6l 

crete  the  spicules  which  form  the  skeleton,  and  still  others  are 
reproductive.  The  spicules  of  which  the  skeleton  is  made  are 
different  in  different  classes  of  sponges.  They  may  be  cal- 
careous, siliceous,  or  horny.  The  sponge  of  commerce  illus- 
trates the  last  class.  The  spicules  may  be  isolated  and  inde- 
pendent, or  become  fused  into  a  continuous  skeleton.  It  is 
the  skeletal  part  which  prevents  the  otherwise  soft  animal 
from  becoming  collapsed  into  a  shapeless  mass,  and  thus  en- 
ables the  cavities,  by  means  of  which  nutrition  is  effected,  to 
be  kept  open.  It  is  the  variety  in  the  skeletons,  too,  which 
gives  the  diversity  of  form  seen  in  the  individuals  of  different 
species. 

205.  Nutrition. — The  food  of  sponges  is  essentially  similar 
to  that  of  the  single-celled  Protozoa.     It  is  carried  in  by  the 
water  currents,  which  enter  the  pores,  pass  along  the  canals 
lined  with  flagellate  entoderm  into  the  cloaca,  and  from  there 
reach  the  exterior  by  way  of  the  osculum.    The-iood  particles 
are  taken  up  principally  by  the  entoderm  cells  liriing  the  radial 
chambers  and  by  the  amoeboid  cells  which  belong ' to  the  mesen- 
chyma.    In  these  cells  digestion  takes  place  as  in  Amoeba.    The 
indigestible  parts  of  the  food  are  returned  to  the  current  and 
are  eliminated  through  the  osculum.     There  is  no  circulation. 
The  digested  food  diffuses  from  cell  to  cell  or  is  carried  by  the 
amoeboid  cells.     Respiration  occurs  through  all  the  cells  which 
are  in  contact  with  the  water. 

206.  Sensation  and  Motion. — Sponges  are  fixed  and  vege- 
tative in  their  adult  life,  and  show  very  little  of  the  more 
active  functions.    In  addition  to  the  ciliated  and  amoeboid  cells 
already  described,   the  pores  may  be  closed  in  response  to 
stimulus.      Both   nervous  and  muscular  elements  have  been 
described  as  occurring  in  these  regions,  but  there  is  some  ques- 
tion as  to  the  degree  of  their  structural  differentiation. 

207.  Reproduction  by  outgrowth  or  budding  is  common. 
In  this  way  large  colonies  arise  from  a  single  individual.    New 
colonies  may  arise,  especially  in  the  fresh-water  sponges,  by 

12 


162 


ZOOLOGY. 


the  separation  of  gemmules  or  groups  of  cells  produced  asex- 
ually  within  the  mesenchyma.  These,  after  a  period  of  rest 
escape  and  produce  new  individuals.  Sexual  reproduction  also 


FIG.  77- 


FIG.  78. 


Tins. 


FIG.  77.  Diagrams  showing  the  arrangement  of  the  radiating  canals  in  two  types  of 
sponges:  A,  Ascon  type;  B,  Sycon  type;  C,  a  portion  (#)  of  the  latter,  more  highly 
magnified,  showing  character  of  the  three  layers,  ec.,  ectoderm;  ent,  entoderm  (flat- 
tened layer);  en2,  flagellate  entoderm;  e.p.,  external  pores;  f.c.,  flagellate  chambers 
of  the  radiating  canals;  i.p.,  internal  pores;  mes.,  mesenchyma;  r.c.,  radiating  canals; 
D,  two  flagellate  cells  more  highly  magnified.  After  Korschelt  and  Heider. 

Questions  on  the  figures. — Trace  the  relation  of  ectoderm  to  the 
entoderm  in  these  two  types?  Compare  these  with  illustrations  in  refer- 
ence texts.  Is  there  any  way  of  accounting  for  this  disproportionate 
growth  of  the  entoderm  ?  What  are  the  apparent  functions  of  the  flagellate, 
collared  epithelium?  What  structures  are  to  be  found  in  the  mesenchyma 
in  sponges? 

FIG.  78.     Axinella  polypoides,   showing  numerous  oscula.     After  Schmidt. 

Questions  on  the  figure. — What  are  the  principal  external  differences 
between  Axinella  and  Leucandra  (Fig.  74)  ?  How  many  individuals  are 
represented  here?  What  are  the  grounds  for  your  answer?  Compare  this 
with  the  skeleton  of  the  sponge  of  commerce. 


PORIFERA.  163 

occurs  in  all  sponges.  The  ova  and  sperm  are  developed  in 
the  mesenchymatous  layer.  The  male  and  female  cells  orig- 
inate from  the  same  individual  (hermaphroditism) .  Usually 
however  the  sexes  mature  at  different  times. 

208.  Development. — Fertilization  of  the  ovum  and  early 
cleavage  take  place  in  the  mesenchyma  near  the   incurrent 
canals,  by  means  of  which  the  spermatozoa  find  entrance. 
Cleavage  is  total  and  for  the  most  part  equal  (see  §  51),  pro- 
ducing an  oval  blastula  which  swims  freely  by  means  of  cilia 
or  flagella.    While  there  are  some  peculiar  features  about  the 
gastrulation,  a  gastrula  or  two-layered  embryo  is  ultimately 
formed.     At  this  stage  the  embryo  settles  to  the  bottom  and 
becomes  attached  by  the  end  containing  the  blastopore,  which 
thus  becomes  obliterated   (Fig.  75,  bl).     An  excurrent  pore 
breaks  through  at  the  opposite  end,  and  the  numerous  incur- 
rent pores  are  formed  at  the  sides.     The  mesenchyma  seems 
to  be  formed  by  cells  which  migrate  from  the  other  layers 
into  the  segmentation  cavity,  thus  filling  it.     The  entoderm 
outpockets  into  the  mesenchyma,  establishing  connection  with 
the  ingrowing  ectoderm,  thus   forming  the  incurrent  canals 
(see  Fig.  76).     In  most  species  the  process  is  more  complex 
than  that  described  here. 

209.  Classification. 

The  divisions  of  the  group  Porifera  are  made  on  the  basis  of  the  dif- 
ferences in  the  skeleton.  Two  principal  classes  may  be  recognized,  as 
follows : 

I.  Calcarea. — Sponges  in  which  the  skeleton  is  composed  of  calcareous 
spicules.    Laboratory  type, — Grantia. 

II.  Non-Calcarea. — Sponges  with  glassy    (siliceous)    spicules,  or  with 
horny  (spongin)  fibres,  or  with  merely  a  gelatinous  mesenchyma.     Labo- 
ratory types: — the  fresh-water  sponge;  the  commercial  sponge. 

210.  Sponges  are  chiefly  marine  animals,  and  flourish  in 
all  the  seas  and  at  any  depth.     The  larger  horny  sponges  of 
which  the  bath  sponge  is  the  skeleton  are  found  in  the  warmer 
seas,  and  in  relatively  shallow  water.     By  reason  of  their 
budding  and  branching,  the  sponges  form  immense  colonies 
or  beds,  and  many  other  forms  of  life  associate  with  them  in 


164  ZOOLOGY. 

varying  degrees  in  intimacy.  Fossil  sponges,  apparently  of 
the  same  general  characteristics  as  those  now  living,  are  found 
in  very  early  geological  strata. 

211.  Supplementary  Library  Studies. 

1.  Economic  value  of  sponges.     Sponge  fisheries.    The  mode  of  pre- 
paring sponges  for  market. 

2.  What  arguments  may  be   advanced  for  considering  the  sponges   as 
colonial  Protozoa?     What  is  the  conclusive  argument  for  regarding  them 
as  Metazoa? 

3.  By  comparing  the  figures  of  sponges  found  in  your  reference  books, 
note  the  different  degrees  of  .development  of  the  passages  lined  with  ento- 
derm  and  ectoderm  in  the  walls  of  various  species. 

4.  In  what  special  ways  do  sponges  become  adapted  to  the  conditions 
in  which  they  are  situated?     Effect  of  rapid  currents  on  them?     Of  quiet 
water?     Of  muddy  water? 


CHAPTER    XII. 

PHYLUM  III.— CCELENTERATA.     (HYDROIDS,  CORALS,  JELLY- 
FISHES,  ETC.) 

LABORATORY  EXERCISES. 

212.  Hydra. — Hydras  are  small  tubular  animals  found  in 
permanent  fresh-water  pools,  attached  to  submerged  leaves, 
twigs,  algse,  etc.  They  are  somewhat  difficult  to  recognize 
when  disturbed  because  they  contract  into  small  rounded 
masses,  close  against  the  supporting  object.  Promising  ma- 
terials should  be  collected  from  several  ponds,  and  placed  in 
shallow  vessels  (a  white- ware  dish  is  good),  and  in  a  short 
time  the  hydras  will  become  extended.  The  green  hydra 
(H.  mridis)  is  perhaps  more  common  and  hardier,  but  is  not 
so  satisfactory  for  general  laboratory  work  as  the  brown 
(H.  fuse  a),  because  it  is  less  transparent. 

I.  Study  the  living  animal  in  a  glass  jar  (tumbler). 

Is  it  free  or  attached?  What  happens  if  it  is  freed  from  its 
attachment?  Is  it  lighter  or  heavier  than  the  water?  Evi- 
dences. Can  it  move  from  one  portion  of  the  vessel  to  an- 
other? If  so,  does  it  become  detached?  Watch  same  individ- 
uals from  day  to  day.  What  is  its  position  in  the  water? 
If  the  vessel  containing  hydras  be  placed  near  the  window,  at 
which  side  of  the  vessel  do  the  animals  become  collected? 
When  the  animals  are  stretched  out  at  their  greatest  length, 
touch  lightly  the  tip  of  one  of  the  tentacles.  Touch  the  body. 
Repeat  the  experiment  until  you  are  sure  of  your  results. 
Note  and  explain  as  well  as  you  can  the  results.  Of  what 
degree  of  contraction  is  the  animal  capable?  Do  you  notice 
any  contractions  or  motions  of  parts,  when  the  hydra  is  un- 
disturbed? What  seems  to  be  the  purpose  of  the  motions? 
Evidences?  Bring  a  piece  of  meat  the  size  of  a  pin-head  or  a 

Daphnia  or  Cyclops  in  contact  with  the  tip  of  a  tentacle  and 

165 


I 66  ZOOLOGY. 

note  the  results.  How  do  the  other  tentacles  behave?  Place 
a  food-particle  directly  at  the  base  of  the  tentacles.  How  is 
it  swallowed?  How  long  does  it  take?  What  becomes  of  it? 
How  long  does  it  remain  in  the  body?  Classify  the  results 
which  you  have  attained,  under  the  following  heads : — motion 
and  locomotion,  nutrition,  sensation.  Devise  still  other  ex- 
periments to  test  special  points  which  you  desire  to  know. 

2.  General  Structure. — Transfer   a   living   animal   to   the 
slide,  covering  it  with  a  drop  or  two  of  water.     Observe  with 
a  low  power  without  cover-glass.     Draw  carefully  in  outline 
everything  studied. 

Note  body  regions : 

Foot  (attached  end). 

Column. 

Tentacles,  position,  number  (examine  several  specimens). 

Hypostome,  surrounded  by  the  tentacles. 

Mouth. 

To  what  extent  do  these  regions  vary  in  their  dimensions 
during  the  different  stages  of  contraction  of  the  hydra? 
Would  you  say  there  is  any  distinct  symmetry?  Which  is  the 
main  axis?  Is  there  any  indication  of  an  internal  (gastro- 
vascular)  cavity?  What  is  its  extent?  Are  the  tentacles  solid 
structures?  Evidences?  Are  there  any  buds  in  your  speci- 
men? Relation  to  the  parent?  To  what  extent  do  different 
parts  of  the  body  do  different  work? 

3.  Microscopic    Structure. — Cover    with    a    cover-glass    supported  by 
objects  as  thick  as  the  animal.     Study  with  a  higher  power.    Verify  the 
points  studied  above.     Follow  the  gastro-vascular  cavity  more  fully.  Is 
there  an  aboral  opening? 

Body  wall. 

Ectoderm,  or  outer  layer  of  cells. 
Entoderm,  or  inner  layer  of  cells. 

Determine  the  extent  of  each  layer.  Are  they  continued  into  the  ten- 
tacles? What  differences  do  you  find  in  the  thickness  of  the  layers  and 
in  the  shape  and  character  of  the  cells  of  each  layer  in  the  various  parts 
of  the  body?  Is  there  anything  between  the  ectoderm  and  entoderm? 

In  the  ectoderm,  especially  in  the  knobs  on  the  tentacles,  find  highly 
refractive  oval  bodies,  the  nettle  capsules.  Irrigate  with  a  drop  of  acetic 
acid,  and  watch  the  tentacle  all  the  while.  What  changes  have  occurred 


CCELENTERATA.  167 

in  the  nettle  cells?     [A  whole  animal  stained  and  mounted  may  be  studied 
profitably  in  comparison  with  the  preceding.] 

4.  Histology  from  Sections. — If  the  teacher  is  not  equipped  for  im- 
bedding and  sectioning  objects,  and  desires  to  carry  this  work  further, 
stained  and  mounted  sections  of  Hydras  and  most  of  the  other  prepared 
sections  suggested  in  this  book  can  be  secured  for  a  reasonable  sum  by 
applying  to  any  of  the  large  laboratories.     By  comparison  of  longitudinal 
and  transverse  sections  verify  your  observations  concerning  the  extent  of 
ectoderm   and   entoderm.     What   occurs  between   the   layers?     Study  the 
shape  and  arrangement  of  the  cells  in  both  layers.     Compare  as  to  size. 
What  is  the  relation  of  the  nettle  cells  to  the  other  ectodermal  cells? 

5.  Histology   from   Maceration  Preparations. — Place   a   specimen   in   a 
watch  glass,  and  draw  away  some  of  the  water  with  a  pipette.    When  the 
Hydra  is  well  extended,  pour  over  it  an  aqueous  solution  of  hot  corrosive 
sublimate.     Rinse  and  place  in  Miiller's  fluid  or  15%  alcohol  for  24  hours. 
Take  a  portion  of  the  body  and  place  on  a  slide  in  a  drop  of  glycerine 
and  water.     Cover,   and  tap   the  cover-glass  very  gently  with  a  needle. 
The  cells  thus  become  separated,  and  their  shape  may  more  readily  be 
seen.     Instructions  for  staining  may  be  found  in  texts  on  histology. 

Study  the  nettle  cells,  the  ectodermal  cells,  the  entoderm,  and  the 
gland  cells  of  the  foot  and  gullet. 

213.  For  comparison  with  Hydra  the  teacher  should  secure  some  alco- 
holic material  of  some  of  the  marine  hydroids,  as  Pennaria,  Obelia  or 
Campanularia.     A  few  slides  should  be  secured  bearing  whole  mounts  and 
sections  properly  stained. 

The  following  points  should  be  studied  briefly:  Relation  between  indi- 
viduals in  the  colonies, — branching.  W7hat  classes  of  individuals  are  dis- 
coverable, i.  e.,  how  do  the  different  branches  end?  Is  there  any  cover- 
ing to  the  softer  portions?  Tentacles;  are  they  present?  If  so,  what  is 
their  arrangement?  Hypostome?  Mouth?  Is  there  a  gastro-vascular 
cavity?  Ectoderm?  Entoderm?  Call  attention  to  polymorphism  among 
the  polyps  or  sooids. 

214.  Metridium    (Sea-anemone). — If  lack  of   appropriations   will   not 
allow  the  purchase  of  sufficient  material  for  class  work,  the  teacher  should 
have  at  least  a  few  well  hardened  and  preserved  specimens  of  sea-ane- 
mone.   From  these  should  be  made  a  series  of  cross-sections  from  various 
parts   of  the  body,  with   a   thickness   of   one-eighth   to   one-fourth   inch. 
These  sections  may  be  fastened  to  cards  or  to  plates  of  mica  by  thread 
or  fine  wire  and  kept  in  preserving  fluid.     One  specimen^should  be  split 
lengthwise,   and  one  left  whole.     Four  or  five  specimens  could  thus  be 
used  from  year  to  year  until  more  abundant  supplies  are  obtained. 

The  following  studies  should  be  made.     Make  drawings  to  illustrate 
all  points  made  out. 
i.  General  Form. 

Base,  or  aboral  disc  (the  end  attached  during  life). 
Column. 


I 68  ZOOLOGY. 

Oral  disc :  zone  of  tentacles ;  intermediate  zone ;  lip-zone ;  mouth ; 
siphonoglyphs  (grooves  in  the  angles  of  the  mouth),— number? 

2.  Transverse  Sections. 
Body  wall. 

(Esophagus;  does  it  appear  in  all  the  sections?     Siphonoglyphs? 

Mesenteries.  How  is  the  oesophagus  held  in  position?  What  dif- 
ferences do  you  find  in  the  mesenteries?  They  are  described  as 
complete  (or  primary),  and  incomplete  (or  secondary,  tertiary, 
etc.). 

Show  by  a  diagram  the  number  and  arrangement  of  them,  especially 
of  the  primary.  Are  they  in  pairs?  Notice  the  inter-mesenteric 
chambers.  Can  you  find  the  muscular  thickenings  in  the  cut  mes- 
enteries? Sketch  their  position.  Compare  with  conditions  figured 
in  various  text-books. 

3.  Longitudinal  Section. 

Complete  your  study  of  the  structures  mentioned  above. 
Compare  the  complete  and  incomplete  mesenteries. 
Identify : 

Mesenteric  filaments   (on  free  edge  of  mesenteries). 
Genital  glands   (developed  in  the  substance  of  the  mesentery  near 

the  edge). 

Ostia,  or  ring  canal;  openings  through  the  mesenteries  by  means 
of  which  the  mesenterial  chambers   communicate  with  one  an- 
other. 
Are  the  tentacles  solid  or  hollow? 

4.  General  Considerations. 

Make  diagrams  in  longitudinal  and  transverse  view  to  show  the  dis- 
tribution and  connection  of  the  cavities  of  the  body.  Is  the  mouth  the 
only  opening  into  the  cavity?  Describe  the  symmetry  of  the  anemone. 
Is  it  radial  or  bilateral?  Give  reasons  for  your  answer. 

215.  Oculina    (or   other   branching   coral). — Study   the   branches    and 
note    the    position    of    the    polyps.     Is    the    arrangement    orderly?     If    so, 
describe. 

Note  with  a  hand  lens  the  arrangement  of  the  septa,  which  grow  be- 
tween the  fleshy  mesenteries  of  the  coral.  Compare  their  arrangement 
with  that  of  the  mesenteries  of  anemone. 

DESCRIPTIVE  TEXT. 

216.  ^ '"MMflp^MMfc^10  SPC    Tes  an(l  the  coelenterates 
in  the  same  group  on  account  of  the  typical  barrel  shape,  the 
absence  of  a  true  ccelom  or  body  cavity,  the  somewhat  similar 
character  and  origin  of  the  middle  mass  (mesenchyma),  and 
the  agreement  of  the  principal  axis  of  the  adult  with  that  of 
the  gastrula.    In  the  coelenterates  however  there  are  no  lateral 


CCELENTERATA. 


i69 


pores.  The  principal  opening  serves  as  a  real  mouth  as  well 
as  vent  for  the  voiding  of  undigested  matter,  whereas  in 
sponges  it  is  not  a  mouth  in  any  sense.  In  general  the  indi- 
vidual even  in  the  colonial  forms  of  ccelenterates  is  more  dis- 
tinctly an  individual  than  in  the  sponges.  The  division  of 
labor  among  the  parts  and  the  interdependence  of  parts  is 
rather  greater  than  among  the  sponges. 

FIG.  79- 


f— -  x 


H. 


-vnf. 


FIG.  79.     A,   Longitudinal   section  through   the  body  ofHydra    (diagrammatic).     B, 
small  portion  of  the  wall  more  highly,  fagnifie.l.     b,   bud;   ect.,   ectoderm: 
derm;  f,  foot;  fl.,  flagellum;  g.v.,  gastio-vascular  cavity;  m.,  moutnjTn^^rmesenchyma 
(noncellular) ;   m.f.,  muscular  processes  of  the   ectodermal  cells;   n,   nettling  cells;   n', 
same,  exploded;  nu.,  nucleus;  t,  tentacle;  v,  vacuole. 

Questions  on  the  figures. — Ho\v  many  cellular  layers  are  to  be  dis- 
tinguished in  Hydra?  What  differentiations  -^ire  represented  in  the 
ectoderm  in  different  regions  ?  In  the  entoderm  ?  \V"hat  is  the  relation  of 
the  bud  to  the  adult?  Why  is  the  cavity  called  a  gastro-vascular  cavity? 
How  is  contraction  effected  in  Hydra? 


17°  ZOOLOGY. 

217.  General  Characters. 

1.  A  single  system  of  internal  chambers   (gastro-vascular 
cavity)   in  which  digestion  and  circulation  both  occur.     No 
coelom. 

2.  Parts  radially  arranged  about  an  oral-aboral  axis.     Ten- 
tacles usually  occur  at  the  oral  pole  (Figs.  80,  83). 

3.  A   supporting   layer   or   mass    (mesenchyma)    between 
ectoderm  and  entoderm,  sometimes  without  cells.    More  often 
cells  of  various  kinds  occur,  which  have  migrated  from  the 
other  layers. 

4.  Nettle  cells  are  found  in  practically  the  whole  group 
(Fig.  81). 

5.  Nerve  cells  (sensory)  and  muscle  cells  both  occur. 

6.  Reproduction  by  non-sexual  methods  is  prevalent.     This- 
often  alternates  regularly  with  the  sexual.    The  individuals  of 
the  two  generations  may  be  very  different  in  appearance  and 
habits. 

7.  Wholly  aquatic;  chiefly  marine. 

218.  General   Survey. — The   group   of    Coelenterata   em- 
braces animals  very  diverse  in  general  appearance,  which  may 
nevertheless  be  reduced  to  two  types.     The  first  and  most 
primitive  is  the  tubular  hydroid  type.     This  is  sessile  and  is 
essentially  a  gastrula,  at  the  unattached  end  of  which  occurs 
the  mouth,  usually  surrounded  by  tentacles.     The  cavity  of 
the  tentacles   is  continuous  with  the  gastro-vascular  cavity 
(Fig.  79).     Of  this  type  we  may  distinguish  two  conditions: 
(i)  in  which  the  individuals  (polyps)  occur  singly  (Hydra), 
or  if  in  colonies,  the  various  individuals  have  the  same  form 
(as  the  corals)  ;  (2)  colonial  forms  in  which  the  individuals 
making  up  the  colony  are  very  different  (as  the  Siphonophora) , 
embracing  open-mouthed  nutritive  individuals,  mouthless  re- 
productive polyps,  protective  polyps  abundantly  supplied  with 
nettle-cells,  bladder-like  supporting  polyps,  etc.  (Figs.  84,  85). 
The  extreme  conditions  of    (i).  and   (2)    are  connected  by 
types    possessing    intermediate    degrees    of    polymorphism. 
Though  the  individual  polyps  are  attached,  the  whole  colony 


CCELENTERATA. 


FIG.  80.  Sections  of  types  of  Ccelenterates  (diagrammatic) :  i  (longitudinal)  and  2 
(transverse)  of  a  tubular  hydroid;  3,  Sea  Anemone  (longitudinal);  4,  same  (transverse, 
at  the  level  of  the  upper  dotted  line) ;  5,  same  (transverse,  at  the  level  of  the  lower 
dotted  line);  6,  longitudinal  or  vertical  section  of  a  Medusa;  7,  transverse  section 
of  same  at  the  level  of  the  dotted  line.  The  continuous  line  is  ectoderm,  the  broken 
line,  entoderm,  and  the  stippled  portion,  mesenchyma.  c.c.,  circular  canal;  g,  gullet; 
g.v.,  gastrovascular  cavity;  m,  mouth;  ma.,  manubrium;  mes.,  mesentery;  mes.1,  direc- 
tive mesentery;  o,  ostium;  r.c.,  radial  canal;  t,  tentacle;  v,  velum. 

Questions  on  the  figures. — By  a  careful  comparison  of  the  diagrams 
what  points  of  similarity  do  you  find  in  these  three  types?  What  are  the 
principal  points  of  difference?  Examine  similar  diagrams  in  other  texts. 
Why  is  Ccelenterate  an  appropriate  name  for  all? 

may  float  freely.  The  second  type  is  the  active  felly-fish,  or 
medusoid  (bell)  type.  The  medusae,  though  varying  greatly 
as  to  details  agree  in  having  a  shape  comparable  to  that  of 
an  umbrella  or  a  bell.  The  convex  surface  is  normally  the 
upper  surface.  At  the  margin  of  the  umbrella  are  tentacles — 
often  very  numerous,  and  frequently  much  elongated.  In  the. 
middle  of  the  concave  surface  is  a  projection,  at  the  lower  end  - 
of  which  is  the  mouth-opening.  The  gullet  leads  from  the- 
mouth  into  a  cavity  in  the  central  portion  of  the  body  of  the 
bell  (gastro -vascular  cavity).  From  the  central  cavity 


172  ZOOLOGY. 

radiating  passages  run  through  the  substance  of  the  bell  to 
the  margin  where  they  may  communicate  with  a  circular  canal 
which  passes  around  the  bell  near  the  bases  of  the  tentacles. 
This  whole  internal  cavity  is  lined  with  entoderm,  and  there- 
fore no  portion  of  it  represents  a  ccelom,  but  is  merely  a  much- 
modified  digestive  tract  (Fig.  80,  6). 

The  bell  is  comparable  to  an  inverted  polyp  in  which  the 
main  axis  has  become  much  shortened,  accompanied  by  a 
thickening  of  the  body  in  the  direction  of  the  other  axes.1 
The  gastro-vascular  cavity  is  further  modified  by  the  increase 
of  the  mesenchyma  of  the  aboral  disc  and  by  a  union  of  the 
oral  and  aboral  walls  of  the  cavity  in  certain  regions.  The 
large  chambers  between  the  mesenteries  in  such  forms  as  the 
sea-anemone  thus  become  limited  to  small  radial  canals.  Fre- 
quently both  of  these  types  are  found  in  the  life  history  of  the 
individuals  of  a  single  species.  The  tubular  colonial  polyp 
produces,  by  asexual  processes  such  as  budding  or  fission, — the 
bell  or  medusoid  forms  which  are  sexuaU  These  may  remain 
attached  or  become  free  swimming.  They  produce  ova  or 
spermatozoa,  or  both,  and  from  the  sexual  union  of  these 
elements  the  non-sexual  tubular  polyp  is  again  produced.  This 
regular  alternation  of  sexual  and  sexless  individuals  is  known 
as  alternation  of  generation.  In  some  forms,  however,  the 
polyp  has  no  corresponding  bell  (as  in  hydra;  corals;  sea- 
anemone),  and  for  some  bells  (as  in  some  large  pelagic  me- 
dusae) there  is  no  corresponding  polyp  stage. 

219.  The  nutritive  processes  in  the  Ccelenterata  are 
marked  by  relative  simplicity.  Food,  consisting  mainly  of 
small  organisms  and  organic  debris,  is  taken  into  the  mouth 
often  with  the  assistance  of  tentacles.  The  tentacles  are  fre- 
quently armed  with  numerous  special  cells  in  which  are  de- 
veloped capsules  containing  long  stinging  threads,  with  barbs 
or  poisonous  tips.  These  may  be  everted  and  possibly  their 
action  brings  about  a  partial  paralysis  of  the  prey.  They 

'See  Text-Book  of  Zoology,  Parker  and  Haswell,  Vol.  I,  p.  127,  Fig. 
89- 


CCELENTERATA. 


serve  also  as  organs  of  defense  (Fig.  81).  Digestion  and 
circulation  both  take  place  in  a  general  cavity  (gastro-vascu- 
lar)  lined  with  entodcrm.  In  other  words  the  circulatory 

FIG.  Si. 


—  c 


-nu. 


FIG.  81.  Nettling  cells  of  Hydra  (after  Schmeil),  A,  unexploded;  B,  exploded,  b, 
barbs;  c,  the  nettling  cell  in  which  the  nettling  organ  is  developed;  en.,  the  cnidocil  or 
"trigger";  cp.,  the  capsule  or  nettling  organ;  f,  the  nettling  filament  or  lasso;  n,  neck 
of  the  capsule;  nu.,  nucleus  of  the  cell. 

Questions  on  the  figure. — Compare  the  parts  of  the  nettling  organ 
before  and  after  explosion  and  note  the  difference  in  position.  How  would 
the  barbs  in  the  neck  of  the  capsule  behave  as  it  is  forced  inside  out  by 
the  compression  of  the  capsule?  Find  from  your  reference  literature  the 
nature  of  the  fluid  secreted  on  the  inside  of  the  lasso. 


J74  ZOOLOGY. 

function  in  this  group  is  not  differentiated  from  the  digestive. 
In  the  colonial  forms  the  gastro-vascular  cavity  of  the  various 
polyps  in  the  colony  may  be  directly  continuous  (Fig.  85). 
In  the  medusa,  the  corals,  and  forms  like  anemone,  the  cavity 
L-  much  more  complicated  than  in  the  tubular  hydroids,  on 
account  of  the  mesenteries,  ^toie  entoderm  seems  to  take  up 
food  from  the  gastro-vascular  cavity,  in  part  at  least,  by  means 
of  the  amoeboid  action  of  some  of  the  entodermic  cells. 
Pseudopodia  are  formed,  and  particles  are  directly  taken  into 
the  body  of  the  cell.  Special  gland  cells  also  occur  in  the 
entoderm,  by  the  secretions  of  which  the  food  undergoes 
changes  preparatory  to  absorption.  There  is  no  anal  opening. 
Undigested  remnants  are  eliminated  at  the  mouth.  (  Respira- 
tion— the  exchange  of  carbon  dioxide  for  oxygen — takes  place 
by  means  of  the  individual  cells  of  the  body  layers,  though  it 
is  probable  that  it  takes  place  more  satisfactorily  in  the  thin- 
walled,  more  actively  moving  tentacles.  Excretion  is  like- 
wise a  general  body  function. 

220.  Motion. — All  the  Coslenterata  are  supplied  with  con- 
tractile fibres.     Many  of  these  are  modified  ectodermal  or 
entodermal  cells  rather  than  true  mesoderm    (Fig.   79,  B). 
The  fibres  run  both  longitudinally  and  transversely.     In  the 
more  active  types  cross-striate  fibres  may  occur.    The  attached 
(polyp)  forms  have  well-developed  longitudinal  fibres  in  the 
body-wall  and  the  mesenteries,  which  enable  the  soft  parts 
of  the  animal  to  be  drawn  close  to  the  supporting  object.    In 
the  medusoid  types  locomotion  is  effected  by  rhythmic  c6n- 
tractions  of  the  bell  as  a  whole.     By  this  means  the  water  is 
expelled  from  the  cavity  of  the  bell,  and  the  reaction  forces 
the  animal  forward. 

221.  Support. — The  attached  colonial  forms  (corals,  sea- 
fans,  etc.)  usually  possess  a  skeleton  of  calcareous  or  horny 
matter  commonly  secreted  by  the  ectoderm.    Each  polyp  con- 
tributes a  portion  to  the  common  skeleton — the  corallum.    The 
corallum  differs  greatly  in  form  in  the  different  species.    This 


CCELENTERATA.  175 

depends  on  the  law  of  budding  or  non-sexual  reproduction 
of  the  polyps,  and  the  activity  shown  by  the  individual  in 
secreting.  In  some  cases  single  polyps  produce  a  skeleton  (cup- 
corals).  The  coral  reefs  of  tropical  seas  are  illustrations  of 
the  power  of  corals  to  form  and  excrete  carbonate  of  lime. 
Much  of  the  lime-stone  of  the  earth's  crust  shows  that  corals 
assisted  in  its  formation. 

222.  Sensation. — The  nerve  cells  may  be  scattered  diffusely 
over  the  surface  of  the  body  with  a  mesh- work  of  fibrils  to 
connect  them  with  the  muscular  and  nettle  cells  and  with  each 
other,  as  in  Hydra.     In  some  other  polyp-forms  there  is  more 
differentiation  of  cells  and  fibres,  but  the  elements  are  still 
scattered.     In  the  more  active  types  there  is  a  collection  of 
the  cells  either  as  a  connected  ring,  or  in  groups,  in  the  ten- 
tacle-bearing rim  of  the  animal.     Associated  with  this  collec- 
tion of  the  nervous  material  into  a  kind  of  nervous  centre, 
there  are  often  special  areas  of  sensory  epithelium,  or  sense 
organs,  developed  from  the  ectoderm.     It  is  not  wholly  clear 
what  kinds  of  stimuli  they  are  suited  to  receive  although  they 
are  designated  as  "  eye  spots,"  or  as  "  auditory  "  or  "  olfac- 
tory "  pits.     The  tactile  sense  is  undoubtedly  present  and  the 
chemical  sense  (taste  or  smell),  although  no  special  organs 
are  apparent.     Otocysts  (see  §  108)  are  found  in  the  cteno- 
phores  and  in  some  medusae,  land  apparently  function  chiefly  as 
organs  of  equilibration. 

223.  Reproduction  and   Development. — The  occurrence 
of  both  sexual  and  asexual  methods  of  reproduction  has  al- 
ready been  mentioned  (§218).     It  is  by  the  latter  method  that 
colonies  are  normally  produced  and  a  given  locality  well  occu- 
pied by  the  species.    By  means  of  the  sexual  method  dispersion 
is  effected,  and  new  regions  are  occupied.    The  ova  and  sper- 
matozoa develop  in  special  gonads  (ovaries  or  testes)  derived 
either  from  the  ectoderm  or  the  entoderm.     The  sexual  cells 
usually  escape  into  the  gastro-vascular  cavity  and  reach  the 
outside  by  way  of  the  mouth.     As  a  rule  the  sexes  occur  in 


176 


ZOOLOGY. 


separate  individuals.  After  fertilization  cleavage  is  total  but 
sometimes  not  equal.  A  blastula  is  formed  which  is  often 
converted  into  a  peculiar,  free  swimming,  ciliated  larva 
(planula),  consisting  of  a  two-layered  sac  with  no  opening. 
This  condition  may  arise  by  the  closing  up  of  an  ordinary  two- 
layered  gastrula  (as  in  Aurelia).  In  other  cases  the  entoderm 


— o 


tnt 


FIG.  82.  Diagrams  illustrating  development  in  some  of  the  hydroid  types.  A, 
blastula  in  which  the  entoderm  (ent.)  is  produced  by  proliferation  from  ectoderm  (ect.), 
B,  ciliated  planula  formed  by  the  continuance  of  this  process.  A  split  in  the  entoderm 
furnishes  the  beginning  of  the  gastrovascular  cavity  (g)  of  the  adult.  C,  more 
mature  condition,  in  which  the  planula  has  become  fixed:  f,  foot  or  attached  end; 
o,  oral  or  free  end  at  which  the  tentacles  and  mouth  will  be  developed. 

Questions  on  the  figures. — How  does  this  blastula  differ  from  the 
typical  blastula  in  the  formation  of  entoderm?  What  is  a  planula?  Is  a 
gastrula  formed?  After  an  opening  forms  at  the  oral  end  what  likeness 
is  there  in  the  adult  to  a  gastrula?  What  changes  would  C  need  to 
undergo  to  become  essentially  similar  to  Hydra? 

may  be  formed  by  cells  budding  into  and  finally  lining  or  even 
filling  the  segmentation  cavity  of  an  ordinary  blastula  (Fig. 
82),  resulting  in  a  quite  similar  condition.  The  planula  after 
a  brief  free  life  becomes  attached  by  one  pole  and  becomes 
elongated;  a  mouth  surrounded  by  tentacles  is  formed  at 
the  other.  Thus  it  assumes  the  typical  polyp  form.  In  nearly 
all  species  the  polyps  may  produce  new  individuals  by  buds 
either  from  the  wall  of  the  polyp  or  from  special  organs 
(stolons,  or  runners).  If,  when  these  are  mature,  they  sepa- 
rate from  the  parent  no  colony  is  formed.  More  commonly 


CCELENTERATA.  1 77 

the  daughters  remain  in  association  with  the  parent.  The 
medusoid  individual, — often  of  a  very  much  simpler  type  than 
that  described  above  (§  218), — may  be  produced  in  a  similar 
\vay  from  a  bud.  It  usually  breaks  its  attachment  with  the 
parent  stock  and  becomes  free-swimming. 

224.  Classification. — The  following  classes  of  Coelenterata  may  be 
recognized. 

Class  I.  Hydrozoa. — Hydrozoa  are  Ccelenterates  with  two  cell-layers 
(ectoderm  and  entoderm),  between  which  there  is  a  supporting  layer  -(the 
mesoglcea')  non-cellular  in  structure.  The  reproductive  cells  arise  chiefly 
from  the  ectoderm.  The  life  cycle  may  consist  of  polyps  alone  (Hydra}  ; 
or  of  medusae  alone;  or  of  both  in  one  life  history  (Campanularia,  Pen- 
naria,  Obelia}.  Medusoid  forms  may  be  free  or  attached.  The  gastro- 
vascular  cavity  is  not  divided  by  mesenteries.  Here  are  included  all  the 
rather  scarce  fresh-water  ccelenterates,  many  tubular  marine  forms  some- 
what similar  to  Hydra,  and  the  much  diversified  colonies  of  the  Siphon- 
ophora  (as  the  Portuguese  Man-of-War,  found  in  mid-ocean,  especially 
in  the  region  of  the  Gulf  Stream).  See  Figs.  84,  85. 

Class  II.  Scyphozoa. — Ccelenterates  in  which  tntf  mesenchyma  con- 
tains cellular  elements.  The  reproductive  cells  arise  from  the  entoderm 
and  escape  into  the  digestive  cavity.  Chiefly  medusoid  forms,  though'  in 
some  the  bell-form  alternates  with  a  polyp  stage.  Types:  Aurelia  and 
the  larger  jelly-fishes.  The  majority  of  the  Scyphozoa  swim  on  the  sur- 
face of  the  ocean;  some  are  found  at  considerable  depths.  Many  of 
•them  are  very  large  and  handsome.  An  especially  interesting  fact  in 
connection  with  the  development  of  such  a  type  as  Aurelia  is  that  its 
polyp  (known  as  the  Scyphistoma)  is  intermediate  in  its  characteristics 
between  the  polyps  of  the  Hydrozoa  and  those  of  the  Actinozoa.  The 
Scyphistoma  has  four  ridges  which  partly  separate  the  gastro-vascular 
cavity  as  do  the  mesenteries  in  the  Actinozoa. 

Class  III.  Actinozoa. — Ccelenterates  with  only  the  polyp  form.  Cells 
in  the  mesenchyma*  There  is  a  well-developed  ectodermic  gullet  (sto- 
moHaeum).  The  gastro-vascular  cavity  is  more  or  less  completely  divided 
into  chambers  by  mesenteries.  Sexual '  cells  entodermal.  A  skeleton  of 
calcareous  or  horny  material  often  present. 

Types :  Sea-anemones ;  sea-fans  and  corals.  The  sea-anemones  or 
sea-roses  are  common  on  rocks  and  other  objects  just  below  low-water 
mark.  Though  attached,  they  have  some  power  of  gradually  changing 
their  position.  Species  of  sea-anemones  are  known  in  which  the  indi- 
viduals are  as  much  as  two  feet  in  diameter,  though  polyps  of  the  colonial 
forms  are  usually  very  small.  . 

Class  IV.  Ctenophora  ("comb-bearers"}. — The  Ctenophora  are  free- 
swimming,  pear-shaped  jelly-fishes,  never  occurring  in  colonies,  and  not 
associated  with  a  polyp  stage.  They  bear  eight  rows  of  vibratile  plates 
composed  of  cilia,  which  function  as  locomotor  and  possibly  as  respiratory 
13 


i78 


ZOOLOGY. 


organs,  and  suggest  the  name  of  the  group.  There  is  a  well-developed 
stomodseum.  The  gastro-vascular  canal  branches  from  this  and  is  much 
divided,  one  division  lying  under  each  row  of  combs.  There  are  two 
small  aboral  openings  to  the  digestive  canal  known  as  excretory  pores. 
The  mesenchyma  is  well  developed. 

225.  Notes  on  Ccelenterates. — The  food  of  Ccelenterates 
consists  largely  of  organic  debris  broken  up  by  the  waves,  and 
of  small  animals  and  plants  captured  by  the  tentacles.  The 
attached  forms  flourish  best  in  the  comparatively  shallow 

FIG.  83. 


—  t 


FIG.  83.  Hydractina  Echinata,  after  Hincks.  c,  the  ccenosarc,  forming  an  incrusta- 
tion over  the  object  on  which  it  lives;  n,  nutritive  polyps;  r,  reproductive  polyps,  bear- 
ing buds  in  which  are  ova;  t,  tentacles. 

Questions  on  the  figure. — How  many  types  of  individuals  seem  to  be 
represented?  What  evidence  of  budding  do  you  see  in  the  species?  What 
is  the  ccenosarc?  What  is  its  nature  in  Hydractina?  What  can  you  find 
concerning  the  habits  of  the  members  of  the  genus  ?  How  does  this  colony 
compare  with  that  in  Fig.  84? 

water  near  the  shore.  Food  is  especially  abundant  in  such 
regions  and  hence  the  passive  animals  are  more  successful 
here  than  elsewhere.  Hydractina  (Fig.  83)  and  even  the  sea- 
anemone  form  interesting  partnerships  with  the  hermit-crab. 


CGELENTERATA. 


I79 


The  polyps  cover  up  the  shell  occupied  by  the  crab,  thus  con- 
cealing it  from  its  enemies  and  its  prey.  In  return  the  polyps 
doubtless  profit  by  a  share  of  the  food  broken  to  pieces  by  the 

FIG.  84. 


FIG.  84.     Physalia,  the  Portuguese   Man-of-war.     After  Agassiz. 

Questions  on  the  figure. — For  what  is  this  animal  remarkable?  To 
what  group  of  ccelenterates  does  it  belong?  Compare  Huxley's  figure  of 
the  same  animal  (see  Parker  and  Haswell's  Zoology,  Vol.  I,  p.  152,  and 
other  reference  texts).  What  various  types  of  polyps  are  represented  in 
the  colony?  Compare  with  Fig.  85. 

crab,  as  well  as  by  the  change  of  place  as  the  crab  moves  about 
in  search  of  food.  Some  anemones  have  living  algae  in  their 
entoderm  cells  which  seem  to  help  supply  the  animal  with 
oxygen  in  return  for  foods  of  other  kinds. 


i8o 


s  5 


r.z* 


s.b 


FIG.  85.  A  very  diagrammatic  and  generalized  illustration  of  a  complex  ccelenterate 
colony.  The  shaded  portion  represents  the  gastro-vascular  cavity.  The  light  portion,  the 
body  tissues,  b.,  a  bell-like  individual  developed  into  an  air-bladder;  m.,  mouth;  n.s., 
nutritive  individual;  p.z.,  protective  individual;  r.s.1,  r.z.2,  r.s.3,  different  types  of  repro- 
ductive individuals;  s.b.,  swimming  bell;  t,  tentacles,  which  are  sensory  and  protec- 
tive structures.  After  Lang. 

Questions  on  the  figure.— What  is  meant  by  "generalized"  above? 
How  does  such  a  polymorphic  colony  as  this  differ  from  a  highly  organ- 
ized individual?  In  what  respect  is  it  similar  to  an  individual?  What  is 
the  function  of  the  gastro-vascular  system?  What  is  the  gain  in  its  wide 
distribution  through  the  colony?  How  do  the  siphonophora  differ  from 
the  other  colonial  ccelenterates  ? 


CCELENTERATA.  l8l 

Many  interesting  experiments  have  been  performed  on 
members  of  this  group  illustrating  the  power  of  regenerating 
lost  parts.  Many  of  the  polyps  have  been  shown  to  have  this 
power  and  even  the  medusae  may  become  perfect  animals  again 
after  having  lost  very  considerable  portions  of  their  structure. 
Hydra,  one  of  the  simplest  members  of  the  group,  is  most 
famous  for  its  power  of  regaining  its  original  form,  no  mat- 
ter to  what  sort  of  mutilation  it  has  been  subjected.  As  long 
as  there  is  a  piece  of  the  trunk  of  appreciable  size  containing 
both  ectoderm  and  entoderm  it  may  regenerate  the  whole 
animal,  stalk,  mouth,  tentacles,  and  all,  under  favorable  con- 
ditions. 

Nothing  about  the  Ccelenterates  is  more  interesting  to  the 
zoologist  than  the  way  in  which  the  individuals  in  the  poly- 
morphic colonies  (as  in  the  Siphonophora)  come  to  do  the 
work  done  by  special  organs  in  the  higher  Metazoa. 

226.  Supplementary  Studies,  for  field  and  library. 

1.  Make  a  list  of  all  the  places  where  Hydra  may  be  found 
in  your  locality. 

2.  Can   you   find    an   account   of    any   other    fresh- water 
Coelenterata? 

3.  What  facts  can  you  find  concerning  the  power  of  re- 
generation in  Hydra  or  other  Ccelenterates  ? 

4.  Coral  reefs:  kinds  and  mode  of  formation.     Conditions 
of  life  necessary  to  the  reef-forming  corals. 

5.  Polyp  colonies.     Show,  by  reference  to  all  the  speci- 
mens and  figures  you  can  find,  where  the  newest  bud  appears 
and  how  this  helps  determine  the  shape  of  the  colony. 

6.  Polymorphism  and  division  of  labor  in  polyp  colonies. 

7.  Corals  in  geological  time. 

8.  Sense  organs  among  Ccelenterates. 

9.  Alternation  of  generation  in  Obelia.     In  Aurelia. 

10.  The  symmetry  of  the  Ccelenterates. 


l82  ZOOLOGY. 

11.  The  structure,  position  and  uses  of  the  nettling  cells  in 
the  phylum. 

12.  Study  the  polyp  of  Aurelia  (Scyphistoma)   from  de- 
scriptions and  cuts,  and  show  in  what  respects  it  seems  to 
stand  intermediate  between  the  Hydrozoa  and  the  Actinozoa. 


CHAPTER    XIII. 
UNSEGMENTED    WORMS    (FLAT- WORMS,    THREAD- WORMS,    ROTIFERS, 

POLYZOA,  ETC.). 

227.  It  seems  desirable,   for  the  sake  of  convenience  and 
in  order  to  prevent  a  confusing  array  of  details,  to  embrace 
under  this  head  a  number  of  groups  of  animals  which  do  not 
have  very  much  in  common  except  their  place  of  uncertainty 
in  the  animal  kingdom.     They  are  not  to  be  considered  as 
forming  a  phylum  of  animals,  although  in  the  past  they  have 
often  been  included  by  authors  with  the  Annulata  (Chapter 
XV)  under  the  head  of  Vermes.    There  is  abundant  evidence 
indeed  to  enable  one  to  believe  that  four  or  five  distinct  phyla 
are  here  included.     Each  of  these  groups,  however,  has  mem- 
bers which  bear  more  or  less  striking  resemblances  to  animals 
belonging  to  the  recognized  phyla,  especially  to  embryonic 
stages  of  them.    These  facts  render  them  of  the  greatest  pos- 
sible interest  to  the  zoologist,  because  they  furnish  grounds 
for  the  hope  that,  through  the  study  of  this  heterogeneous 
assemblage,  the  origin  and  kinships  of  all  the  other  phyla  may 
be  made  more  clear.    The  same  facts  make  them  unfit  objects 
for  extended  study  in  elementary  classes. 

228.  Points  of  General  Resemblance. — In  external  form 
these  animals  differ  very  greatly.     They  may  vary  from  a 
cylindrical  or  even  a  globular  form  to  a  thin  ribbon-shape. 
They  agree  for  the  most  part,  however,  in  having  a  main 
axis  which  in  the  free-swimming  forms  is  usually  horizontal 
in  position,  the  anterior  end  of  which  is  structurally  distin- 
guishable  from  the  posterior.     There  is  usually  a  distinct 
bilateral  symmetry  (see  §  116)  which  takes  the  place  of  the 
radial  symmetry  found  in  the  Coelenterates.    In  some  types  of 
the  Coelenterates  there  are  certain  suggestions  of  bilateral 
symmetry  but  never  to  the  complete  exclusion  of  the  radial. 

183 


1 84  ZOOLOGY. 

For  the  first  time  is  found  an  assemblage  of  multi- 
cellular  animals  whose  individuals  move  with  one  end  con- 
tinually foremost  and  one  of  the  body  surfaces  continually 
up  and  the  other  down.  This  is  a  distinct  gain  in  organization 
and  accompanies  a  more  active  life.  The  Polyzoa  are  at- 
tached in  adult  life  and  have  lost  this  symmetry,  and  many 
of  the  Rotifers,  while  having  definite  anterior  and  posterior 
ends,  have  lost  their  right-left  symmetry  in  part,  but  the  em- 
bryonic stages  of  these  are  in  many  respects  similar  to  the 
more  typical  forms.  By  saying  that  these  animals  are  un- 
segmented  it  is  meant  that  in  a  distinct  individual  there  is  not 
usually  a  linear  series  of  equivalent  body-parts  or  metameres. 
There  are  however  several  types  which  reproduce  new  in- 
dividuals by  transverse  division  ("fission").  These  new 
individuals  may  remain  together,  temporarily  at  least,  in  a 
chain,  as  in  Microstomum  (Fig.  89)  or  the  tape-worm  (Fig. 
91),  forming  a  strobila.  In  this  condition  there  is  a  repeti- 
tion of  all  the  essential  organs  in  each  of  the  "  segments." 
Some  authors  regard  this  process  of  strobilation  as  the  con- 
dition from  which  the  ordinary  segmentation,  as  seen  in  the 
Annulata,  has  arisen,  by  the  adhesion  and  gradual  differen- 
tiation of  the  originally  similar  individuals.  The  animals  of 
these  groups  agree  in  the  fact  that  the  third  or  mesodermal 
layer  of  tissue  becomes  more  important  than  it  is  among  the 
Ccelenterates.  In  addition  to  this  the  mesoderm  often,  though 
not  universally,  splits,  forming  a  coelom  or  body  cavity 
(§56)  wholly  separate  from  the  digestive  tract.  The  coelom 
is  lined  with  mesoderm.  All  the  animal  phyla  above  the 
Ccelenterates  possess  this  character  in  some  measure  and  on 
this  account  are  called  coclomata.  These  animals  further  agree 
with  those  above  them  in  the  scale  of  development  in  possess- 
ing a  system  of  excretory  tubules  which  connect  the  ccelom, 
or  the  mesodermal  tissue  if  there  is  no  ccelom,  with  the  out- 
side world.  This  is  sometimes  spoken  of  as  the  "  water- 
vascular  "  system  to  distinguish  it  from  the  blood  vessels. 


UNSEGMENTED    WORMS.  185 

229.  Laboratory   Exercises. — An   extended   laboratory   study  of  these 
groups    is   not   desirable,  yet  the   teacher   should   secure   enough   material 
representing  the  various  included  phyla  to  enable  the  student  to  justify 
the   separation  of  these  uncertain   forms   from   the   more   exactly  defined 
phyla ;  and  to  show  him  how  ill-defined  is  the  assemblage  which  we  have 
thus  brought  together.     The  Tape-worm  of  man  may  sometimes  be  secured 
from  physicians,  and  other  species  of   Tania  are  found  not   infrequently 
as    intestinal   parasites    in   cats,   dogs,   or   other   animals   dissected   in   the 
laboratory.     The  general  form,  the  method  of  attachment  to  the  host,  the 
progressive  development  of  the  proglottides  or  "  segments  ",  and  the  dif- 
ference between  these  segments  and  those  of  the  Earth-worm  should  be 
noted.     Permanent  whole  mounts   of  a  mature  proglottis  may  be   made, 
showing  the  embryos  in  the  uterus.     Demonstrations  of  the  structure  of 
the  proglottis  may  be  given  by  properly  prepared  transverse  sections,  if 
the  equipment  and  time  allow. 

An  hour's  work  may  profitably  be  devoted  to  the  study  of  some  one 
or  more  of  the  common  Rotifers,  which  may  be  found  in  water  taken 
from  the  stagnant  pools  in  which  there  is  much  decaying  matter.  They 
are  microscopic  animals  and  are  to  be  recognized  by  the  possession  of 
discs  at  the  anterior  end,  which  present  the  appearance  of  rotating  wheels 
because  of  a  rhythmic  action  of  the  cilia.  Make  sketches  showing  the 
change  of  shape  which  the  animal  undergoes.  How  is  the  change  effected? 
How  is  locomotion  accomplished  ?  What  evidences  have  you  of  its  ability 
to  receive  stimuli  and  to  respond  to  them?  How  does  it  get  food?.  Can 
you  trace  the  digestive  tract  in  the  body  of  the  animal  ?  Notice  the  con- 
tracting object  just  back  of  the  mouth.  What  conclusions  do  you  reach 
as  to  its  function?  Give  your  evidences.  Verify  by  consulting  some  text- 
book. Can  you  prove  from  what  you  see  that  this  is  not  a  single-celled 
animal  like  Stentor?  The  student  should  be  cautioned  against  taking 
these  specimens  as  closely  typical  of  the  whole  group  of  Rotifers,  since 
there  is  very  great  variety  of  form  among  them. 

Planarians  often  appear  in  the  laboratory  in  water  containing  an 
abundance  of  decomposing  organic  matter,  taken  from  ponds  and  foul 
streams.  The  most  important  points  to  be  noticed  are  their  general  form, 
the  method  of  locomotion,  sensitiveness  to  stimuli,  and  life  habits.  Non- 
sexual  reproduction  by  fission  is  frequent  among  them. 

The  Polyzoa  occur  as  tufts  of  many  minute  animals  in  colonies  at- 
tached to  objects  in  the  water.  Plumatella  is  a  rather  common  fresh- 
water form  and  makes  a  beautiful  demonstration  to  illustrate  the  ordi- 
nary physiological  processes,  as  motion,  feeding,  the  action  of  the  diges- 
tive tract  in  churning  the  food,  sensitiveness  to  stimulus  and  the  like. 
Schools  near  the  sea-shore  will  find  an  abundance  of  marine  material  for 
the  comparison  of  the  colonial  forms  of  different  species  of  Polyzoa,  since 
they  are  more  common  in  salt  than  in  fresh  water. 

230.  Classification  and  Description.    Phylum  Platyhelminthes  (Flat- 
worms). — In  the   worms   of   this  phylum   the   body   is   flattened   or  com- 


i86 


ZOOLOGY. 


pressed  in  a  dorso-ventral  direction,  and  from  this  fact  the  name  is  given. 
They  are  soft-bodied  animals  without  any  true  skeleton.  There  is  no 
body  cavity  and  no  true  blood-vascular  system.  The  space  which  would 
be  given  to  such  structures  is  filled  with  a  spongy  connective  tissue. 

FIG.  86. 


FIG.  86.  Diagram  ot  a  Turbellarian,  showing  the  general  arrangement  of  the  nervous 
structures  and  one  of  the  modes  of  occurrence  of  the  excretory  tubules,  which  in  this 
case  open  separately  into  the  pharynx,  on  the  ventral  side  of  the  animal,  b.,  brain;  e, 
eye-spots;  ex,  excretory  canals  consisting  of  a  transverse  portion  passing  from  the 
mouth  toward  the  dorsal  side  (see  also  Fig.  87),  and  longitudinal  tubes  which  branch 
into  the  capillary  vessels  terminating  in  f,  the  flame  cells;  lc.,  lateral  nerve  cords;  m, 
mouth. 

Questions  on  the  figure. — Compare  this  figure  with  the  next  and 
identify  the  structures  shown  in  both.  What  other  positions  of  the  mouth 
do  you  discover  in  the  Turbellaria,  as  figured  in  reference  texts?  What 
other  arrangement  of  the  excretory  canals  and  pores  ? 

Through  this  body-mass  run  the  minute  tubes  of  the  excretory  or  water- 
vascular  system  (Fig.  86,  ex.},  often  terminating  internally  in  special  cells 
(flame  cells,  Fig.  88).  These  tubes  have  external  pores.  By  means 
of  this  system  of  organs  waste  products,  probably  of  a  nitrogenous  nature, 
are  eliminated  from  the  tissues.  The  digestive  tract  may  be  wholly  want- 
ing as  in  the  Cestodes,  or  a  simple  or  forked  sac,  or  a  central  sac  with 
lateral  branches.  It  is  blind,  *.  e.,  has  only  the  oral  opening.  In  the  more 
complicated  types  of  stomach  the  much-branched  sac  serves  the  function 
of  carrying  the  digested  food  to  all  parts  of  the  body.  Many  of  these 
forms  are  parasitic  and  in  consequence  the  organs  referred  to  are  often 


UNSEGMENTED    WORMS. 


i87 


very  much  simplified  and  degenerate.  The  digestive  tract,  for  example, 
may  be  entirely  lost  Reproduction  by  transverse  division  is  not  uncom- 
mon. By  this  method  strobilae  or  chains  of  more  or  less  closely  connected 
individuals  occur.  The  sexual  organs  are  exceedingly  complex,  particu- 
larly in  the  parasitic  members  of  the  group  (Fig.  92).  The  develop- 
ment is  in  some  instances  direct,  in  others  indirect.  The  principal  classes 
are  the  Turbellaria,  Trematodes  and  Cestodes. 


FIG.  87. 


FIG.  88. 


FIG.  87.  Diagram  of  transverse  section  of  a  Turbellarian  through  the  region  of  the 
mouth,  d.m.,  dermo-muscular  wall  containing  longitudinal  fibres;  ex,  excretory  system; 
f,  flame  cells;  g,  gut;  I.e.,  lateral  nerve  cord;  m,  mouth;  m.f.,  muscle  fibres;  ph., 
pharynx;  t,  testis;  u,  uterus;  y,  yolk  glands. 

Questions  on  the  figure. — Determine  with  care  the  relation  of  this 
to  the  preceding  diagram  and  identify  the  common  structures.  What  new 
structures  are  represented  here?  What  would  be  their  position  in  the 
former  figure?  The  great  range  in  position  of  the  muscle  fibres  and  the 
spongy  character  of  the  body  contribute  to  what  powers? 

FIG.  88.  Diagram  of  flame  cell,  the  internal  terminus  of  the  excretory  tubules,  c, 
cilia  lining  the  tubule;  f,  special  cilia  constituting  the  Home;  n,  nucleus  of  flame  cell; 
p,  cell  processes;  v,  vacuole  or  cavity  in  cell  communicating  with  the  capillary 
tubules  (0. 

Questions  on  the  figure.— What  is  the  function  of  the  cell  itself? 
Of  the  flame? 

Class  i.  Turbellaria  (Planarians,  etc.). — These  are  mostly  small  non- 
parasitic  Platyhelminthes  with  a  ciliated  ectoderm.  They  are  chiefly 
aquatic  and  are  carnivorous.  The  ventral  mouth  may  be  anterior,  pos- 
terior, or  median  in  position.  It  opens  into  a  muscular  eversible  pharynx, 
which  may  be  used  to  assist  in  locomotion.  The  digestive  tract  may  be 
simple  or  very  much  branched.  The  brain  consists  of  a  pair  of  ganglia 
in  the  anterior  region.  From  the  brain  lateral  nerve  cords  pass  backward 


i88 


ZOOLOGY. 


through  the  body.  The  excretory  organs  (Figs.  86,  87)  usually  consist  of 
two  or  more  longitudinal  tubes  which,  open  on  the  exterior  separately  or 
by  a  common  orifice.  The  position  of  the  opening  varies  very  much  in 
the  different  orders.  The  tubules  are  much  branched  interiorly  and  pene- 
trate the  soft  tissues  of  the  body  as  minute  capillaries  with  thin  walls. 
They  terminate  in  cells  of  special  structure  which  are  excretory  in  func- 
tion. A  group  of  cilia  (the  flame,  Fig.  88,  /)  helps  in  creating  a  current 
in  the  capillary  tubes.  The  lining  of  the  tube  may  also  be  supplied  with 
cilia.  The  Turbellaria  have  remarkable  powers  of  regenerating  lost  por- 
tions. Experiments  show  that  very  small  portions  of  an  individual  will, 
under  favorable  conditions,  reproduce  all  the  parts  of  a  complete  animal. 
In  habit  they  may  be  terrestrial,  fresh-water  or  marine.  They  vary  in 
size  from  microscopic  fresh-water  forms  to  a  length  of  six  inches  or  more 
in  the  case  of  the  marine  and  land  types  (Figs.  86-89). 


FIG.  89.  Diagrammatic  sagittal  section  of  Microstomum,  showing  a  chain  of  four 
zooids  produced  by  fission,  b,  brain  of  the  original  zooid  (the  exponents  indicating 
corresponding  structures  of  the  more  recently  formed  zooids) ;  c,.  ciliated  pit;  d,  dis- 
sepiments indicating  different  stages  in  the  separation  of  the  zooids;  e,  eyespot;  ent, 
entoderm;  g,  gut;  gl.,  glandular  cells  about  the  mouth;  m,  mouth  of  the  original  worm. 

Questions  on  the  figure. — What  various  evidences  can  be  found  of 
the  relative  age  of  the  zooids?  Is  the  mouth  formed  apparently  from 
entoderm  or  ectoderm?  Is  the  gut  a  blind  sac?  What  incidents  seem 
necessary  when  this  chain  separates  at  the  oldest  plane  of  division,  and 
forms  two  chains,  in  order  that  each  may  be  like  the  parent?  How  is 
this  like  segmentation  in  annulates  (see  Fig.  99)?  How  unlike? 

Class  2.  Trematoda. — The  Trematodes  are  small,  usually  parasitic, 
Platyhelminthes.  The  ectoderm  is  provided  with  a  protective  "  cuticle " 
and  is  consequently  destitute  of  cilia.  They  possess  a  well-developed  and 
often  much-branched  digestive  sac,  which  has  only  one  opening — the 
mouth.  Usually  one  or  more  sucking  discs  are  present.  By  means  of 
these  the  parasite  attaches  itself  to  the  host.  The  nervous  and  excretory 
systems  are  similar  in  general  to  those  of  the  Turbellaria,  but  are  some- 
what better  developed  and  more  complex.  In  those  members  of  the  class 
which  are  external  parasites  there  is  usually  no  metamorphosis  in  the 
development.  In  the  internal  parasites,  as  the  Liver-fluke  of  the  Sheep, 


A  series  of  diagrams  illustrating  the  life  cycle  in  the  LIVER  FLUKE  (Distomum). 
After  Thomas,  Leuckart,  and  others.  A,  egg  in  its  case;  B,  early  embryo,  still  in 
case;  C,  free-swimming  ciliated  embryo;  D,  same  after  encysting  in  tissues  of  snail 
{sporocyst) ;  E,  sporocyst  at  later  stage  producing  by  internal,  non-sexual  processes 
new  sporocysts,  and  re  dice  (r)  which  break  from  the  sporocyst  and  lead  an  inde- 
pendent life  of  their  own  in  the  tissues  of  the  snail;  F,  a  mature  redia  producing 
within  itself  new  generations  of  redise,  and  a  new  type  of  larva,  cercarice  which 
escape  by  a  birth-pore  (&./>.)  and  make  their  way  into  the  water;  G,  cercaria;  H, 
same  after  losing  its  tail  and  becoming  encysted;  I,  the  young  fluke  in  the  liver  of 
the  sheep,  where  it  becomes  sexually  mature  and  produces  perhaps  500,000  new 
eggs,  b,  brain;  b.p.,  birth  pore;  c,  cercaria;  c.m.,  cell  masses, — embryos  formed  non- 
sexually  within  sporocysts  and  rediae;  e,  eye-spots;  ex.,  excretory  tubules  and  pore 
(only  the  posterior  portion  shows);  g,  gut;  m,  mouth;  ph,  pharynx;  r,  redia;  s, 
suckers;  sc,  sporocyst;  +,  stages  in  which  non-sexual  reproduction  occurs;  *,  stage  at 
which  sexual  reproduction  occurs. 

Questions  on  the  figures. — In  which  stages  are  eyespots  found?  Num- 
ber and  position  of  the  suckers?  In  which  stages  found?  What  is  the 
result  of  increasing  the  points  at  which  reproduction  occurs  in  the  cycle? 
Is  this  a  combination  of  metamorphosis  and  alternation  of  generation? 
Your  reasons  for  your  answer?  Compare  this  with  the  life  history  of  the 
tape-worm.  Note  the  encysted  stage  by  which  it  passes  from  water  to  its 
host  in  each  instance. 

189 


19°  ZOOLOGY. 

there  is  frequently  a  most  complicated  metamorphosis  coupled  with  an 
alternation  of  sexual  and  non-sexual  generation  (see  §218).  A 
Liver-fluke  (Distomum  hepaticum}  is  found  in  the  bile  ducts  of  the  liver 
of  the  sheep,  where  it  gives  rise  to  a  much-dreaded  disease — "  liver  rot." 
The  eggs  which  are  formed,  fertilized  and  pass  through  the  early  stages 
of  cleavage  here,  pass  out  of  the  bile  ducts  to  the  intestine  and  thence  to 
the  exterior.  If  the  larva  reaches  water  it  develops  into  a  free-swim- 
ming larva  (Fig.  90,  C.),  which  to  insure  further  development  must  bore 
into  the  tissues  of  a  particular  pond-snail  (Limnaa  truncatuld).  It  there 
develops  into  a  kind  of  sac  (sporocyst}  from  the  inner  cells  of  which 
special  cells  are  budded  (Fig.  90,  £).  These  cells  have  the  power  of 
developing  into  embryos  of  a  second  generation  by  cell  division — that 
is  to  say,  non-sexually.  Several  such  non-sexual  reproductions  may  occur 
in  the  body  of  the  snail  (Fig.  go,  -f-).  These  later  generations  'of  larvae 
pass,  often  by  the  death  of  the  snail,  into  the  water,  whence  they  may 
enter  the  alimentary  tract  of  the  sheep  in  drinking.  The  larvae  find  their 
way  to  the  liver  and  develop  there  again  into  the  adult  fluke.  It  is  evi- 
dent that  such  a  form  must  have  immense  powers  of  reproduction,  when 
it  is  considered  that  the  reproduction  takes  place  at  several  points  in  the 
life  cycle  (Fig.  90,  -f  *)•  This  may  be  seen  to  be  a  necessity  to  compensate 
for  the  great  loss  of  life  involved  in  changing  from  host  to  host.  It  is 
said  that  a  single  fluke  may  produce  half  a  million  eggs.  Each  of  these 
which  succeeds  in  reaching  the  host  snail  may  produce  hundreds  of  the 
last  generation  of  asexual  individuals.  The  disease  is  prevalent  only  in 
those  countries  where  this  species  of  Limnaa  occurs.  It  is  much  worse 
in  wet  years.  Millions  of  sheep  have  died  in  England  alone,  in  a  single 
year,  from  the  attacks  of  this  parasite.  Trematode  parasites  are  common 
among  the  vertebrates  and  frequent  most  diverse  organs. 

Class  3.  Cestodes  (Tape-worm,  etc.}. — The  Cestodes  are  internal 
parasites  having  a  complicated  life  history  usually  involving  two  hosts. 
In  the  tissues  of  the  first  host  occurs  the  "  bladder- worm,"  Cysticercus, 
or  embryonic  stage  (Fig.  91,  A};  in  the  intestine  of  a  second  host  the 
strobila  or  adult  tapeworm  (Fig.  91,  C)  is  found.  The  adult  form  has 
no  mouth  or  digestive  tract,  the  animal  taking  its  food  by  absorption  of 
the  digested  material  in  which  it  is  bathed.  The  anterior  end  is  supplied 
with  hooks  or  suckers  by  means  of  which  it  attaches  itself  to  the  intes- 
tinal wall.  Just  behind  this  "head"  is  a  region  in  which  transverse 
division  (Fig.  91,  z;  and  §122)  is  continually  going  on.  This  results 
in  the  continuous  formation  of  new  segments  or  proglottides,  the  older 
ones  being  pushed  further  from  the  head  by  those  newly  formed.  Each 
proglottis  becomes  in  time  a  sexually  mature  hermaphrodite  individual. 
All  stages  of  sexual  maturity  are  found  in  one  strobila  or  colony,  the 
posterior  individuals  being  most  mature.  At  the  posterior  end  of  an  old 
colony  the  proglottides  (Figs.  91, 92)  are  filled  with  the  developing  embryos, 
and  on  breaking  away  from  the  chain  these  brood  cases  pass  with  the 
faecal  matter  from  the  intestine.  In  this  way  it  becomes  possible  for  the 
embryos  to  find  the  way  into  a  new  host.  On  being  swallowed  by  some 


UNSEGMENTED    WORMS. 
FIG.  91. 


>r— -T-— r- 

CDO 


700 


D 


FIG.  91.     Diagram  showing  some  stages  in  the  life  history  of  the  Tapeworm  (Tcenid). 

A,  Cysticercus  or  Bladderworm  stage,  before  the  "  head  "  protrudes  from  the  bladder; 

B,  same,  later  stage;    C,   Strobila,   or  chain  of  proglottides,   many  being   omitted;   D, 
embryo,  such  as  fill  the  uterus  of  the  mature  proglottides.     It  is  protected  by  a  shell. 
b,  bladder;  ex.,  excretory  canals";  g,  genital  pore;  h,  head  or  scolex  provided  with  hooks 
and  suckers  (s)  ;  u,  uterus  in  a  mature  posterior  proglottis;  z,  zone  of  budding  or  seg- 
ment formation.     The  numerals  show  the  approximate  number  of  the  segments,  reckon- 
ing   from   the    front.     Not    more   than    5    per    cent,    of    real    length    of    the    chain    is 
represented. 

Questions  on  the  figure. — What  arguments  do  you  find  from  the 
figure  for  considering  the  strobila  an  individual?  What  for  considering  it 
a  colony?  Where  does  non-sexual  reproduction  occur?  Where  sexual? 
Seek  figures  of  stages  between  D  and  A  in  the  reference  books. 


I92 


ZOOLOGY. 


suitable  animal  they  break  from  their  cysts,  bore  through  the  wall  of  the 
digestive  tract  into  the  tissues.  Here  they  grow,  become  encysted  and  at 
this  stage  develop,  in  anticipation  of  the  needs  of  the  adult  worm,  the  head 
or  scolex  which  remains  attached  to  the  bladder-like  cyst  (Fig.  91,  A,  B). 
Development  stops  at  this  point  unless  the  flesh  of  this  host  is  eaten  by 

FIG.  92. 


FJG.  92.  Diagram  of  a  sexually  mature  proglottis  of  Ttenia.  A,  anterior  end;  e, 
embryos;  ex.,  excretory  canals;  g.p.,  genital  pore;  ov.,  ovaries  (paired);  r.s.,  recep- 
taculum  seminis;  s.g.,  shell  gland;  t,  testes;  ut.,  uterus  filled  with  embryos;  v,  vagina; 
v.d.,  vas  deferens;  y.g.,  yolk  gland. 

Questions  on  the  figure. — Why  is  self-fertilization  possible  in  tape- 
worm? What  is  the  function  of  the  various  portions  of  the  reproductive 
apparatus?  Trace  the  following  steps  and  indicate  where  each  incident 
happens :  formation  of  eggs  and  sperm ;  passage  of  sperm  to  vas  deferens 
and  into  vagina;  storing  of  sperm  in  receptaculum  seminis;  fertilization 
in  the  oviduct ;  addition  of  yolk ;  ovum  covered  with  the  shell  secretion ; 
passage  into  uterus  where  development  proceeds. 

some  other  animal.  When  this  happens  the  bladder  is  thrown  off,  the 
head  becomes  attached  to  the  wall  of  the  intestine  of  the  carnivorous  host, 
and  the  active  formation  of  the  chain  of  proglottides  begins  again.  The 
more  common  Tape-worms  of  man  are  Tania  solium  and  T&nia  saginata. 
The  former  is  more  common  in  Europe  and  is  received  into  the  system 


UNSEGMENTED    WORMS.  193 

by  eating  the  raw  flesh  of  the  pig,  in  which  the  bladder-worm  stage  occurs. 
The  latter  is  obtained  chiefly  from  beef  and  is  more  common  in  America. 
Only  by  adequate  cooking  is  the  danger  of  infection  removed.  The  Amer- 
ican habit  of  eating  beef  rare  contributes  to  the  spread  of  the  pest.  Other 
tape-worms  infest,  as  their  double  host,  the  dog  and  the  rabbit;  the  cat 
and  the  mouse ;  the  shark  and  other  fishes.  The  excretory  system  is  a 
pair  of  continuous  lateral  tubes  with  transverse  connections  in  the  various 
proglottides  (Fig.  92,  ex}.  The  nervous  system  in  the  adult  tape- worm 
includes  a  rather  complex  series  of  loops  containing  nerve-cells,  in  the 
scolex,  with  right  and  left  lateral  lines  of  nervous  tissue  running  the 
length  of  the  strobila.  There  are  numerous  longitudinal,  transverse 
(circular),  and  dorso-ventral  muscle  fibres  passing  through  the  spongy 
tissue  of  the  worm.  There  is  a  well-developed  external  cuticle  which 
helps  protect  the  animal  from  the  action  of  the  digestive  juices  of  the 
host. 

Phylum  Nemathelminthes  (Round-  or  Thread-worms). — Nemathel- 
minthes  are  elongated,  cylindrical  forms  which  taper  at  the  ends.  The 
body  is  covered  by  a  dense  cuticle.  Some  are  aquatic  but  most  are  para- 
sitic at  least  during  a  part  of  their  life.  An  alimentary  tract  is  present 
and  has  both  a  mouth  and  an  anus.  There  is  a  coelom  which  is  not 
divided  into  chambers  and  contains  a  fluid  without  corpuscles.  There  is 
no  circulatory  system  other  than  this.  There  are  no  special  respiratory 
organs.  The  central  nervous  system  consists  of  a  ring  around  the  oesoph- 
agus. This  contains  some  nerve  cells.  From  this  ring  nerves  arise  at 
various  points  and  pass  both  forward  and  backward.  The  chief  posterior 
nerve  is  ventral,  but  there  may  be  also  dorsal  and  lateral  ones.  The  sexes 
are  usually  separate.  Development  is  sometimes  direct,  sometimes  in- 
direct. The  best-known  representatives  arc  the  round-worms  (Ascaris), 
different  species  of  which  are  found  in  the  intestine  of  man,  of  the  pig, 
and  of  the  horse ;  vinegar-"  eels  "  ;  trichina. 

Trichina  is  one  of  the  most  dangerous  of  the  nematode  parasites.  The 
sexually  mature  worm  occurs  in  the  intestine  of  the  rat,  the  pig,  man,  or 
other  mammal.  The  young  are  retained  by  the  mother  in  the  uterus 
until  well  developed.  When  born  the  young  bore  through  the  wall  of 
the  intestine  of  the  host  and  make  their  way  to  the  muscles,  where  they 
become  encysted  and  cause  degeneration  of  the  muscle  fibres  and  often 
other  acute  symptoms  of  the  disease  known  as  trichinosis.  The  larvae 
remain  in  their  cysts  indefinitely  or  until  the  death  of  their  host.  For 
further  development  the  flesh  must  be  eaten.  In  the  intestine  of  the  new 
host  where  the  cyst  is  dissolved  the  adult  condition  is  quickly  reached, 
reproduction  takes  place  again,  the  embryos  migrate  into  the  muscles  and 
the  new  cycle  is  begun.  We  do  not  find  here  the  non-sexual  reproduction 
that  helped  make  the  Liver-fluke  so  prolific,  but  the  reproductive  power 
of  Trichina  is  very  great  without  this.  It  is  estimated  that  an  ounce  of 
"  measly "  pork  may  contain  80,000  cysts  of  Trichina,  and  that  each 
female  produced  from  these  embryos  may  contain  at  one  time  1,000  or 
more  embryos.  During  her  life  she  may  produce  ten  times  this  number. 


ZOOLOGY. 


Thus  the  40,000  females  from  such  a  meal  would  soon  supply  40,000,000 
young  worms  for  the  infection  of  the  muscles,  with  the  ability  of  renew- 
ing the  supply  at  short  periods.  Perfect  cooking  is  the  only  sure  safe- 
guard against  the  possibility  of  infection. 

FIG.  93. 


—-ft 


FIG.  93.  Diagram  of  a  sagittal  section  of  a  Rotifer,  b,  brain;  &/.,  excretory 
bladder;  c,  cloaca,  the  common  opening  of  digestive  and  reproductive  organs;  co, 
ccelom;  e,  eyespot;  ex,  excretory  canal;  /,  flame  cells;  f.g.,  foot  gland;  ft.,  foot;  g,  gut; 
m,  mouth;  m.f.,  longitudinal  muscle  fibres;  mx,  mastax;  o,  ovary;  ph.,  pharynx;  s.g., 
salivary  gland;  t,  tentacle;  tr,  trochus,  or  cilia-bearing  disc. 

Questions  on  the  figure. — What  sets  of  organs  and  functions  are  in- 
dicated in  the  diagram?  Does  this  seem  a  lower  or  higher  form  than  the 
other  types  studied  in  this  chapter?  What  are  your  grounds  for  your 
answer?  What  indications  of  segmentation  are  represented  in  the  figure? 
Is  the  mastax  in  the  stomodaeum  or  mesenteron?  Where  do  the  various 
authors  classify  Rotifers? 


UNSEGMENTED    WORMS.  195 

Phylum  Trochelminthes  (wheel-worms  or  rotifers). — The  Rotifers 
or  wheel-animalcules  are  microscopic  animals.  They  are  usually  bilater- 
ally symmetrical.  The  anterior  end  possesses  a  retractile  disc  supplied 
with  cilia  variously  arranged,  the  rhythmic  motions  of  which  often  give 
the  appearance  of  a  rotating  wheel.  From  this  the  name  of  the  group 
comes.  This  organ  assists  in  locomotion  and  produces  currents  in  the 
water  by  which  food  is  brought  within  reach  of  the  mouth.  There  is  a 
digestive  tract  with  both  mouth  and  anus.  The  pharynx  into  which  the 
mouth  opens  is  provided  with  a  chitinous  grinding  apparatus  (niastax). 
Usually  a  pair  of  digestive  glands  open  into  the  stomach.  The  nervous 
system  is  usually  limited  to  a  single  ganglion  dorsal  to  the  pharynx. 
Eye-spots  and  other  sense  organs,  called  tactile  rods  or  antennae,  are  pres- 
ent. There  is  a  true  coelom  communicating  with  the  exterior  by  means 
of  excretory  tubules.  For  a  diagrammatic  view  of  these  structures  see 
Fig.  93- 

The  sexes  are  distinct  and  are  frequently  very  different  in  appearance. 
The  males  are  often  much  smaller  than  the  females,  are  much  less  numer- 
ous, and  are  often  degenerate.  The  summer  eggs  are  of  two  kinds — 
large  and  small — and  develop  parthenogenetically.  The  large  eggs  pro- 
duce females  and  the  small,  males.  The  winter  eggs  have  a  thick  shell 
and  are  believed  to  require  fertilization  in  order  to  develop.  They  rest 
during  the  winter  and  in  the  spring  develop  into  females.  Development 
is  direct.  The  adult  condition  in  the  Rotifers  suggests  the  larval  (trocho- 
phore)  condition  in  some  Annulata.  There  are  some  traces  of  external 
segmentation  in  the  tail  or  foot  region  in  some  species  and  for  these 
reasons  some  authors  class  the  Rotifers  near  the  Annulata.  Rotifers  are 
aquatic,  being  more  common  in  fresh  water  than  in  the  sea.  They  are 
abundant  in  water-troughs,  gutters,  ponds.  They  are  capable  of  resuming 
activity  after  having  been  dried  up  in  the  mud  for  a  year  or  more.  This 
power  must  be  of  great  value  in  preserving  the  species  as  well  as  in 
spreading  it. 

Phylum  Molluscoidea  (mollusk-like) . — The  two  groups  included  here 
are  quite  diverse  in  general  appearance  and  habit  Their  larval  stages 
have  more  points  in  common  than  the  adult.  There  is  in  the  adult  a 
variously-shaped  tentacle-bearing  ridge  (lophophore)  about  the  mouth. 
-  The  central  nervous  system  consists  of  one  or  two  ganglia  about  the 
oesophagus.  They  have  often  been  grouped  with  the  mollusks  but  authors 
are  agreed  that  much  of  the  seeming  resemblance  to  mollusks  is  super- 
ficial. 

Class  i.  Polysoa  (Bryozoa;  sea-mats;  corallines)  .—The  Polyzoa  are 
colonial  animals  which  resemble  in  general  appearance  some  of  the  com- 
pound hydroids.  The  individual  animals  however  are  very  different  in 
their  structure.  They  are  found  both  in  salt  and  fresh  water.  In  Poly- 
zoa (Fig.  94)  the  digestive  tract  is  sharply  bent,  the  anus  opening  close 
to  the  mouth  either  within  or  outside  the  circle  of  tentacles  (lophophore). 
A  distinct  coelom  is  typically  present.  There  are  no  blood  vessels. 
An  exoskeleton  is  formed  by  the  ectoderm,  by  means  of  which  the  indi- 


196 


ZOOLOGY. 


viduals  of  the  colony  are  held  together.  Each  member  of  the  colony 
may  retire  into  its  own  particular  portion  of  the  exoskeleton,  when  dis- 
turbed, by  the  contraction  of  appropriate  muscles.  The  brain  consists 
of  a  single  ganglion  lying  between  the  mouth  and  anus.  The  two  sexes 
usually  occur  in  the  same  individual.  The  colonies  are  formed  by  budding, 
which  takes  place  in  each  species  in  a  way  that  is  characteristic  of  that 
species.  Thus  it  comes  about  that  the  colonies  differ  as  much  in  general 
appearance  as  their  individuals  do  in  structure. 

FIG.  94. 


FIG.  94.  A  fresh-water  polyzoan,  Plumatella.  From  Parker  and  Haswell,  after 
Allman.  a,  anus;  fu.,  funiculus,  a  band  of  tissue  anchoring  the  intestine  to  the  body 
wall;  g,  ganglion;  int.,  intestine;  m,  mouth;  o,  oesophagus;  r,  reproductive  gland;  rt, 
retractor  muscle;  st,  stomach;  stat,  statoblast;  t,  tentacles. 

Questions  on  the  figure. — Is  this  an  individual  or  a  colony?  What  is 
the  function  of  the  retractor  muscles?  To  what  degree  are  the  polyps 
capable  of  contraction  as  shown  in  the  figure?  The  value  of  this  power? 
What  are  the  statoblasts? 

Class  2.  Brachiopoda  (arm-footed;  lamp-shells}. — Brachiopods  are 
marine  forms  chiefly  of  geological  interest,  as  there  are  at  present  only  a 
few  living  species.  They  were  very  prevalent  in  early  geological  times. 
They  possess  a  bivalved  shell  which  suggests  that  of  the  bivalve  Mollusca 
(as  the  clam).  From  this  external  resemblance  they  have  long  been 
classed  as  mollusks.  The  valves  are  strictly  dorsal  and  ventral  in  the 
Brachiopods,  however;  whereas  in  the  mollusks  they  are  right  and  left. 
Their  shell  is  therefore  no  longer  considered  as  homologous  with  the 
mollusk  shell.  The  internal  structure  is  still  further  removed  from  that 
of  the  clam.  The  digestive  tract  is  often  bent  much  as  in  the  Polyzoa, 
ancl  the  mouth  is  surrounded  by  a  tentacle-bearing  lophophore  (the 


UNSEGMENTED    WORMS.  197 

"arms").  The  lophophore  may  have  a  skeletal  support  which  in  differ- 
ent types  assumes  different  shapes  (loop,  helix,  or  spiral).  A  peduncle 
usually  extrudes  at  the  hinge,  by  means  of  which  the  animal  attaches-  itself 
to  foreign  objects.  The  Brachiopods  are  not  colonial.  The  student  is 
referred  to  the  more  extended  texts  for  illustrations  of  this  group  of 
animals. 

231.  Notes  on  Ecology  and  Distribution. — The  organ- 
isms included  in  this  chapter  represent  the  most  varied  modes 
of  life.  The  Turbellarians  are  free  animals  and  may  be  ter- 
restrial, fresh-water  or  marine;  the  Rotifers  are  as  a  rule 
free-swimming  and  occur  chiefly  in  fresh  water;  the  Polyzoa- 
are  aquatic,  attached,  colonial  forms  but  lead  for  the  most 
part  an  independent  existence,  or  may  occasionally  be  com- 
mensal with  other  types  of  animals;  the  Brachiopods  are 
marine  and  may  be  attached,  but  are  not  colonial ;  the  Trema- 
todes  and  Cestodes  represent  all  kinds  and  degrees  of  para- 
sitism. Even  if  all  these  classes  of  animals  could  be  con- 
sidered akin,  their  habits  of  life  and  their  consequent  adapta- 
tions are  so  various  as  to  produce  the  greatest  range  of  gen- 
eral form  and  special  structure. 

If  we  consider  the  relatively  small  number  of  species  of 
animals  in  these  groups,  the  species  of  the  Platyhelminthes  are 
among  the  most  widely  distributed  of  the  metazoa.  This  is 
true  both  of  the  free  Turbellaria  and  the  parasitic  Trematodes 
and  Cestodes.  There  is  probably  not  a  large  group  of  the 
metazoa  which  escapes  being  the  host  of  one  or  more  of  these 
worms  at  some  stage  of  its  life  history.  The  fact  of  para- 
sitism, the  ability  to  carry  on  the  life  cycle  in  a  series  of  hosts, 
and  the  prevalence  of  the  carnivorous  habit  among  its  hosts 
all  help  the  distribution.  The  organs  more  commonly  infested 
by  the  parasites  are  the  digestive  tube,  the  blood  and  lymphatic 
vessels,  the  ccelomic  cavity  or  other  organs  where  the  nutritive 
fluids  of  the  body  are  abundant.  They  produce  all  sorts  of 
disorders  from  mere  functional  disturbance  (such  as  digestive 
disorders  and  anaemia  from  the  presence  of  the  tape-worm) 
to  the  destruction  of  the  tissues  of  the  organs  involved.  It 
is  very  commonly  true  that  the  adult  or  sexually  mature  in- 


198  ZOOLOGY. 

dividuals  are  produced  in  one  host,  and  the  eggs  or  larvae  pro- 
duced by  them  find  their  way  into  another  species  of  host  where 
a  portion  of  the  development  toward  maturity  occurs.  The 
transfer  of  the  parasite  from  the  second  back  to  the  first  host- 
species  is  necessary  to  complete  the  cycle.  In  some  instances 
there  is  not  a  change  from  one  animal  to  another,  but  merely 
from  one  organ  to  another  in  the  same  animal,  as  in  Tcenia 
murina  of  the  rat.  In  size  the  unsegmented  worms  vary 
from  minute  microscopic  dimensions  to  thirty  feet  in  length 
in  the  tape-worm,  Bothriocephahis  latus.  Some  suggestion  of 
their  importance  to  man  and  the  higher  animals  may  be 
gathered  by  reference  to  the  following  table  (p.  199). 

232.  Supplementary  Studies  for  the  Library. 

1.  In  what  different  ways  are  the  forms  included  in  this 
chapter  classified  in  the  various  text-books  to  which  you  have 
access  ? 

2.  Consider  the  economic  importance  of  the  parasites  in- 
cluded in  this  chapter. 

3.  Make  a  further  study  of  the  life  histories  of  selected 
representatives  of  these  parasites. 

4.  Illustrate  by  means  of  the  unsegmented  worms  the  de- 
generation and  simplification  which  attends  parasitism. 

5.  In  what  various  ways  do  the  intestinal  parasites  in  the 
group  adhere  to  the  walls  of  the  digestive  tract  of  the  host? 

6.  Do  you  think  the  domestic  animals  are  more  or  less 
likely  to  be  attacked  and  suffer  from  these  internal  parasites 
than  the  wild?    What  evidences  would  you  offer   for  your 
view? 

7.  Prepare   for  the  class  a   diagram  of  the  reproductive 
organs  in  the  Tape-worm,  indicating  the  function  of  each  of 
the  portions. 

8.  What  is  meant  by  the  "  dermo-muscular  "  sac  in  worms? 
Its  functions? 

9.  Report  on  the  importance  of  the  Brachiopods  in  early 
geological  time,  with  the  main  structural  features  of  the  class. 


UNSEGMENTED     WORMS. 


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II 


CHAPTER  XIV. 

PHYLUM  IV.-ECHINODERMATA   (STAR-FISH,  SEA-URCHINS,  SAND- 
DOLLARS,  SEA-LILIES). 

LABORATORY  EXERCISES. 

233.  Asterias    (Star-fish).— Both    dry    and    alcoholic,    or 
otherwise  preserved,  materials  should  be  at  hand. 

1.  General  form. 

Central  disc. 

Rays;  number,  form,  size,  etc.  Compare  several  in- 
dividuals. 

Oral  surface  (contains  mouth)  ;  aboral  surface.  Note 
all  the  differences  between  these  surfaces,  both 
in  the  arms  and  the  disc. 

The  axis  of  an  arm  is  known  as  a  radius;  the  space 
between  is  interradial. 

Is  the  body  bilaterally  symmetrical  or  radially  sym- 
metrical? Give  the  reasons  for  your  conclusion. 

2.  External  anatomy. 
Oral  surface. 

Mouth :  position  and  surroundings. 

Ambulacral  groove:  position,  relation  to  the  mouth, 

extent. 
Ambulacral  feet:  how  arranged?    Is  the  foot  hollow 

or  solid?    Pull  off  one,  and  examine  with  lens  or 

low  power  of  the  microscope. 
Aboral  surface. 

Madreporic  body:  position    (radial  or  interradial?), 

shape,  size,  structure. 
Bivium;  trivium  (see  text,  §  237). 
Examine  the  spines  on  both  surfaces  and  determine 

the   arrangement   and   shape   in   different   regions. 

How  are  they  fixed  to  the  body? 

200 


ECHINODERMATA.  2OI 

Pedicellariae  (at  the  base  of  the  spines)  ;  papulae  (soft 

bodies  among  the  spines).     Examine  with  lens. 
Make  an  outline  drawing  of  each   surface,  filling  in  the 
details  of  the  disc  and  one  arm  and  showing  the  points  above 
determined.     Sketch  one  of  each  of  the  various  classes  of 
spines  in  profile. 

3.  Organs  of  the  body-cavity. 

Using  alcoholic  or  other  moist  preparations,  cut  into  one  side  of  an 
arm  of  the  trivium,  making  an  incision  from  near  the  tip  almost  to  the 
disc.  Cut  across  the  back  of  the  arm  near  the  tip  and  make  a  similar 
incision  on  the  other  side.  Lift  the  flap  thus  separated  and  notice  the 
organs  attached  to  it.  The  material  should  be  dissected  under  water  or  50 
per  cent,  alcohol,  or  kept  moistened  therewith. 
Hepatic  cseca ;  extent,  number,  and  attachment. 

Detach  the  hepatic  cseca  from  the  aboral  wall  by  breaking  the  mesen- 
teries, and  treat  all  the  arms  of  the  trivium  as  above. 

Carefully  connect  the  incisions  across  the  interradii  and  remove  the 
entire  aboral  wall  except  that  around  the  madreporic  body  and  that  be- 
tween it  and  the  centre  of  the  disc,  being  careful  to  disturb  none  of  the 
soft  parts.  If  material  is  scarce  the  teacher  should  make  a  few  dissec- 
tions to  be  used  as  demonstrations. 
Notice : 

Body-cavity,  its  extent  and  contents. 

Stomach:  pyloric  (aboral)  portion;  shape,  position.     Are  the  hepatic 
caeca  connected  with  it?   Verify.     (The  stomach  opens  aborally  into 
a  small,  short  rectum  and  anus  usually  very  difficult  of  demon- 
stration.)    Rectal  diverticula?    number  and  position? 
Cardiac  (oral)  portion  of  stomach;  pouches,  number  and  form;  re- 
tractor muscles,  attached  to  the  floor  of  the  arms. 
Mouth ;  peristome. 

Remove  the  hepatic  caeca  from  one  arm  and  find  the  genital  glands, 

which  lie  in  the  floor  of  the  body-cavity.     What  is  their  number 

and  arrangement  ?     At  what  point  do  they  connect  with  the  body 

wall?     Can  you  prove  that  they  communicate  with  the  exterior? 

Ampullae  (on  ventral  floor)  :  determine  if  they  connect  with  ambu- 

lacral  feet. 

Make  three  diagrams  showing  the  position  of  the  organs  thus  far 
studied:  (i)  the  aboral  surface  with  the  wall  removed,  showing 
the  stomach  in  the  disc,  the  hepatic  caeca  in  one  arm,  the  repro- 
ductive bodies  in  a  second,  and  the  ampullae  and  retractor  muscles 
in  a  third;  (2)  a  transverse  section  of  an  arm  about  midway  be- 
tween its  ends;  and  (3)  a  sagittal  section  of  an  arm  continued 
through  the  disc. 


202  ZOOLOGY. 

4.  Ambulacral  system. 

In  a  specimen  from  which  the  preceding  organs  have  been  removed, 
make  a  transverse  section  of  an  arm  about  an  inch  from  the  disc.  Find . 
the  radial  water  canal,  a  small  tube  lying  just  outside  the  skeleton  in 
the  ambulacral  groove.  Force  air  into  it  with  a  blow-pipe,  or  inject  a 
colored  solution  with  a  hypodermic  syringe.  What  other  structures  are 
affected?  Trace  connection  between  radial  canal,  ampulla,  and  ambulacral 
feet.  Compare  the  number  of  ampullae  and  the  number  of  feet.  Follow 
the  radial  canal  toward  the  disc.  How  does  it  terminate? 

From  the  madreporic  body  trace  the  S-shaped  stone  canal  toward  the 
oral  surface.  How  does  it  terminate? 

Ring  canal:  its  position.  Are  there  any  other  structures  (sacs)  in 
communication  with  the  circum-oral  ring-canal  beside  the  stone  canal  and 
the  radial  water-tubes?  form  and  position? 

5.  Nervous  system. 

There  is  a  radial  nerve  (in  the  skin)  superficial  to  the  radial  water 
canal  in  each  arm.  The  radial  nerves  unite  to  form  a  circumoral  nerve 
ring. 

6.  Skeletal  parts. 

Dried  material  and  portions  soaked  for  a  day  or  so  in  a  10  per  cent, 
solution  of  potash  should  be  used  to  supplement  the  alcoholic  specimens. 

Is  the  skeleton  complete,  i.  e.,  are  the  ossicles  in  contact? 

Are  they  .similarly  arranged  on  the  aboral  and  oral  surfaces  ?  Which 
surface  shows  the  greater  differentiation?  Illustrate,  and  find  a  reason 
if  you  can.  How  are  the  ossicles  related  to  the  spines?  to  the  papulse? 
Study  with  some  care  the  ossicles  forming  the  ambulacral  groove,  begin- 
ning at  the  middle  line. 

Ambulacral  rafters:    shape  and  arrangement. 

Ambulacral  pores ;  are  they  in,  or  between,  the  ossicles  ? 

Adambulacral  ossicles  (just  lateral  to  the  former)  ;  how  do  these 
compare  in  number  with  the  ambulacral  ossicles? 

"  Cross-shaped  "  ossicles. 

Which  of  the  above  bear  spines  ?   what  kind  ? 

Place  some  of  the  cleaned  ossicles  in  dilute  hydrochloric  acid.  Result? 
What  is  the  significance  of  this  result? 

7.  Physiological  experiments  are  possible  only  near  the  seashore.     The 
animals  must  be  kept  in  sea  water,  and  studied  soon  after  being  collected. 
When  possible,  locomotion,  the  action  of  the  ambulacral  feet,  feeding,  and 
sensitiveness  should  be  studied.     Do  you  find  any  indications,  among  the 
specimens  provided,  of  the  power  to  renew  a  lost  arm?     With  care  and 
perseverance,  at  the  proper  time  of  year,  the  sexual  elements  may  be  col- 
lected and  the  maturation,  fertilization,  and  cleavage  of  the  ovum  illus- 
trated in  this  group.     Teachers  in  inland  schools  should  procure,  when- 
ever possible,  slides  demonstrating  the  early  development  of  the  star-fish 
or  sea-urchin. 

8.  Compare  briefly  the  external  features  of  other  "  stars "  with  that 
already  studied. 


ECHINODERMATA.  203 

234.  Sea-urchin   (Echinus  or  Arbacid). 

A  few  skeletons  of  sea-urchins  and  sand-dollars  will  be  of  great  value 
in  enabling  the  pupil  to  see  how  the  same  general  plan  of  structure  may 
be  varied  to  meet  different  needs. 

1.  Spines    (if  present)  :  arrangement  and  method  of  attachment.     Are 
they  of  the  same  appearance  and  composition  as  the  skeleton?    Do  you 
find  any  signs  of  the  former  presence  of  ambulacral  feet?     If  so  what, 
and    how    arranged?     Can    they    all   have    the    same    function    as    in    the 
star-fish?    Proofs? 

2.  Ossicles :    Make  out  the  boundaries.     Compare  with  the  condition  in 
the  star-fish.     What  are  the  special  advantages  gained  by  each  arrange- 
ment?    Can    you    find    anything    corresponding    to    ambulacral    ossicles? 
(Look  for  the  pores.)      What  corresponds  to  the  ambulacral  groove  in 
Asteriasf    Identify  the  interambulacral  ossicles.     How  arranged?    What 
is  radial  and  what  interradial  in  the  urchin?   What  in  the  sea-urchin  would 
correspond  to  the  oral  and  aboral  surfaces  in  the  star-fish?    Evidences? 
Find  the  madreporic  body.    Make  a  plot  of  all  the  ossicles  in  this  region, 
noting  the  differences.    Find  the  genital  pores. 

3.  "Aristotle's  lantern"   (the  mouth  apparatus). 

Examine  the  structure  as  a  whole.  How  related  to  the  body?  Study 
the  parts  in  their  relation  to  each  other.  Number,  and  method  of  action? 

DESCRIPTIVE  TEXT. 

235.  The  Echinoderms  form  a  very  distinct  group  of  ani- 
mals, which  in  the  adult  condition  at  least  show  a  decided 
radial  symmetry.     They  possess  a  more  or  less  extensive  cal- 
careous  exo-skeleton   with   outwardly  directed   spines.     The 
star-fishes,    sea-urchins,    brittle    stars,    sea-lilies,    and    sea-cu- 
cumbers are  representatives.     They  are  marine  in  habit  and 
may  be  either  fixed  or  slow-moving.     They  agree  with  the 
Ccelenterates  in  having  radial  symmetry,  and  in  the  absence 
of   a   well-marked   brain   and   other   signs   of   cephalization. 
There  is  considerable  ground  for  believing  that  this  is  an  out- 
come of  their  sluggish  habit,   since  the  larval  condition  is 
bilaterally  symmetrical,  and  radial  symmetry  is  clearly  adapted 
to  a  passive  life.     It  is  difficult  to  determine  the  relationships 
of  the  Echinoderms ;  yet  it  seems  probable  that  their  ancestors 
were  bilateral  forms.     Perhaps  they  should  be  considered  as 
connected  with  the  worms  rather  than  with  the  Ccelenterates. 

236.  General  Characters. 

i.  Larvae  are  bilaterally  symmetrical;  in  the  adult  there  is 


204  ZOOLOGY. 

a  more  or  less  complete  radial  arrangement  of  equivalent  parts, 
usually  on  the  plan  of  five.  In  this  radial  plan  all  the  principal 
sets  of  organs  share:  as  the  nervous,  digestive,  reproductive, 
etc. 

2.  There  is  a  complete  differentiation  of  digestive  tract  and 
body  cavity. 

3.  The    blood-vascular    system    is    partially    differentiated 
from  the  body  cavity,  but  communicates  with  it. 

4.  A  calcareous  exo-skeleton  occurs,  derived  from  the  meso- 
derm.     It  may  consist  of  isolated  spicules  or  united  plates. 
Associated   with   these   are   usually   spines,    from   which   the 
group  is  named. 

5.  A  water- vascular  system, — consisting  of  a  series  of  tubes 
(closed  except  at  one  point),  muscular  sacs   (ampullae)   and 
distensible  feet, — serves  a  locomotor  and  respiratory  function. 

6.  Reproduction  sexual;  development  usually  indirect,  {.  e., 
with  a  metamorphosis.     Reproduction  by  budding  does  not 
occur. 

237.  General  Survey. — The  majority  of  echinodesms  have 
a  central  disc  in  which  is  located  portions  of  the  various  sets 
of  organs.  Ordinarily  there  radiate  from  this  disc  more  or 
less  clearly  defined  rays  or  arms  in  which  lie  radial  outgrowths 
from  the  central  organs.  The  spaces  between  the  rays  (inter- 
radii)  may  be  bridged  by  growth  in  such  a  way  that  the  dis- 
tinction between  rays  and  disc  is  not  marked.  In  crinoids  the 
arms  may  be  much  branched.  The  oral-aboral  axis  is  usually 
pronounced,  often  short,  and  is  vertical  in  position  (asteroids, 
echinoids,  crinoids,  etc.),  though  in  the  sea-cucumbers  (holo- 
thuroids)  it  is  horizontal  and  much  elongated.  Star-fish  are 
flattened  vertically,  as  are  the  sand-dollars;  but  many  of  the 
urchins  (echinoids)  are  dome-shaped.  The  antimeres  are  at 
right  angles  to  this  chief  axis.  In  addition  to  this  dominant 
radial  symmetry,  there  is  seen  even  in  the  adult  a  suggestion 
of  the  bilateral  condition.  The  madreporic  body  generally 
occurs  in  only  one  interradius,  and  a  plane  passing  through 
it  and  splitting  the  opposite  arm  divides  the  body  into  two 


ECHINODERMATA.  205 

FIG.  95. 


FIG.  95.     Starfish,    from   chart   of    Leuckart    and    Nitsche. 

Questions  on  the  figure. — How  would  you  describe  the  symmetry 
of  the  animal?  Identify  and  name,  by  comparison  with  the  diagrams  and 
the  text,  all  the  structures  which  show  in  the  figure.  Compare  this  with 
specimens  or  figures  of  the  common  American  species  and  note  the  chief 
differences. 

symmetrical  halves.  No  other  plane  does  this.  The  two  arms 
embracing  the  madreporic  body  are  known  as  the  bivium,  the 
remaining  three,  the  trivium.  In  some  of  the  echinoids  the 
bilateral  symmetry  becomes  much  more  pronounced  than  in 
star-fish. 

238.  The  integument  consists  of  an  outer  ectodermal  por- 
tion which  is  often  ciliated  (cilia  wanting  in  the  holothuroids 
and  ophiuroids),  and  a  subepithelial,  mesodermic  layer  in 
which  is  developed  the  calcareous  ossicles.  These  may  occur 
as  spicules,  as  rods,  or  as  plates  in  the  various  classes.  Fre- 
quently the  ossicles  bear  spines  which  may  or  may  not  be 


206 


ZOOLOGY. 


movable.  The  spines  are  useful  in  defense  and  locomotion. 
Special  forms  of  spines  known  as  pedicellarice  often  occur 
(asteroids  and  echinoids).  They  consist  of  two-  or  three- 
pronged  pincers  moved  by  muscles.  They  may  be  mounted 
on  short  stalks.  It  is  suggested  that  they  help  clear  the  body 
of  foreign  objects  which  lodge  among  the  spines. 

239.  Digestive  System. — The  mouth  and  anus  usually 
open  at  opposite  poles  of  the  principal  axis  (asteroids,  holo- 
thuroids,  and  some  echinoids).  When  the  axis  is  vertical  the 
mouth  is  usually  directed  downward,  in  the  centre  of  the  oral 
surface,  and  the  anus  occupies  a  more  or  less  central  position 
on  the  upper  or  aboral  surface.  In  some  of  the  echinoids  the 
mouth  or  anus,  or  both,  have  vacated  their  central  position  and 
have  come  to  occupy  opposite  margins  of  the  body.  The  diges- 
tive tract  is  a  simple  tube,  in  the  holothuroids  running  spirally 
through  the  body.  In  the  echinoids  a  similar  condition  is 
found  except  that  it  begins  in  a  complex  masticating  apparatus 


FIG.  96. 


FIG.  96.  Vertical  (sagittal)  section  through  an  arm  and  an  interradius  of  a  Starfish 
(diagrammatic),  a,  anus;  amp.,  ampulla;  c.b.,  circular  blood  vessel;  c.w.,  circular 
water  canal;  co.,  ccelom;  co.e.,  ccelomic  epithelium;  d.b.,  dermal  branchiae;  e,  position 
of  the  eyespot;  ect.,  ectoderm;  ent,  entoderm;  f,  arnbulacral  foot;  g,  ambulacral 
groove;  h,  hepatic  caeca  or  liver;  i,  intestine;  i.e.,  intestinal  caeca;  mes,  mesoderm; 
mo".,  mouth;  m.p.,  madreporic  body;  n.r.,  nerve  ring;  os.,  ossicles  in  mesoderm; 
r.n.,  radial  nerve  band;  r.b.,  radial  blood  vessel;  r.p.,  reproductive  pore;  r.w., 
radial  water  canal;  s.c.,  stone  canal;  sp.,  spines;  z,  lacunar  spaces  in  the  mesoderm. 
(Adapted  from  various  sources.) 


ECHINODERMATA.  2OJ 

of  five  parts  (Aristotle's  lantern).  In  the  asteroids  the  mouth 
opens  by  a  short  oesophagus  into  an  expanded  stomach,  which 
is  divided  into  an  oral,  or  cardiac,  and  a  pyloric  portion  (Fig. 
96).  From  the  pyloric  part  the  narrow  intestine  passes  to 
the  anus.  Outpocketings  (caeca)  may  occur  in  any  of  these 
divisions.  The  most  important  are  the  hepatic  caeca,  which 
are  glandular  in  function. 

240.  The  body  cavity  is  usually  well  developed  both  in  the 
disc  and  in  the  arms,  is  lined  with  a  ciliated  epithelium,  and 
contains  a  fluid  with  amoeboid  corpuscles.    It  is  completely  dis- 
tinct from  the  digestive  cavity.    Thin  outgrowths  of  the  body- 
wall   (papulce  or  branchice)  contain  extensions  of  the  ccelom. 
These  assist  in  respiration. 

241.  Ambulacral  or  Water- vascular  System. — This  sys- 
tem of  tubular  organs  is  peculiar  to  the  echinoderms.     It 
originates  (see  also  248),  in  common  with  the  body  cavity,  as 
an  outgrowth  from  the  archenteron  and  is  to  be  regarded  as 
a  specialized  portion  of  the  body  cavity.     In  some  cases  these 
two  cavities  are  in  communication  in  the  adult.     It  consists 
essentially  of  a  ring-vessel  about  the  mouth  from  which  pass 
radial  tubes,  one  in, each  arm.     From  the  radial  tubes  arise 
lateral    channels    which    communicate    directly    or    through 
bladder-like  ampullae,  with  distensible  feet  which  reach  the 
exterior  by  pores  in  the  skeleton  (Figs.  97,  98).    The  tip  of  the 
foot  may  be  provided  with  a  sucking-disc,  serving  as  a  means 
of  attachment  and  of  locomotion.     Frequently  the  walls  of 
these  feet  are  thin  and  apparently  serve  for  respiration,  and  the 
terminal  "  foot "  at  the  end  of  each  radius  may  be  highly 
modified  to  form  a  sense  organ   (tentacle).     The  feet,  the 
ampullae,  and  even  the  radial  vessels  may  be  wanting.     The 
ring-canal,  in  typical  forms,  communicates  with  the  surround- 
ing sea-water  by  means  of  a  tube  (stone  canal)  which  termi- 
nates in  a  sieve-like  plate,  the  madreporic  body,  through  which 
the  water  enters  the  water- vascular  system.     In  the  majority 
of  the  Holothuroids  the  madreporic  tubes  open  into  the  body 


208 


ZOOLOGY. 


cavity  instead  of  opening  to  the  exterior.  In  consequence  the 
fluid  which  is  found  in  the  water-vascular  system  is  the  same 
as  that  of  the  body  cavity  and  contains  amoeboid  cells.  In 
the  crinoids  also  the  water-vascular  system  communicates 
directly  with  the  ccelom,  but  there  is  no  true  madreporic  canal. 
In  its  stead  is  found  a  system  of  ciliated  water-tubes  in  con- 
nection with  the  ring  canal.  Identify  the  elements  in  the 
water-vascular  system  from  Fig.  98. 

242.  Respiration  occurs  in  connection  with  the  water- 
vascular  system  especially  in  those  forms  in  which  the  ten- 
tacles and  ambulacral  feet  are  possessed  of  thin  walls  (holo- 
thuroids  and  some  echinoids).  In  the  asteroids  and  echinoids 

FIG.  97. 


r.  p.... 


FIG.  97.  Transverse  section  of  the  arm  of  a  Starfish  near  the  disc.  Diagrammatic. 
Lettering  as  in  preceding  figure,  a.r.,  ambulacral  rafter  (ossicle*) ;  ov.,  ovary,  con- 
taining ova. 

Questions  on  Figs.  96  and  97. — What  are  the  principal  sets  of  organs 
represented  in  the  disc  of  the  starfish?  Which  of  these  have  radial  por-? 
tions  going  into  the  arms?  Follow  carefully  the  ectodermal,  entodermal 
and  mesodermal  boundaries.  Locate  and  identify  the  various  structures 
lettered,  and  determine  as  far  as  possible,  whether  the  essential  part  of 
each  is  furnished  by  ectoderm,  entoderm  or  mesoderm.  Is  there  a  coelom? 
Your  evidences?  What  is  the  relation  of  the  water- vascular  cavity  to  the 
coelom,  in  origin? 


ECHINODERMATA. 

there  are  thin  outpocketings  of  the  body- wall,  papulae  or 
branchiae  (Fig.  97,  d.b.),  the  cavity  in  which  is  continuous 
with  the  body-cavity.  The  body  fluids  may  thus  be  aerated 
from  the  water  outside.  In  some  forms  water  is  drawn  into 
special  branching  pockets  (respiratory  tree)  in  the  wall  of  the 
rectum,  and  later  is  forced  out  again. 

243.  Circulation. — The     circulatory    vessels     are    merely 
partly  differentiated  portions  of  the  ccelom  or  body  cavity. 
Our  knowledge  is  by  no  means  complete  but  it  seems  that  in 
none  of  the  groups  is  there  a  complete  separation  of  the  blood 
spaces  from  the  ccelom.     There  are  probably  no  contractile 
hearts.     The  walls  of  the  blood  spaces  may  bear  cilia,  which 
assist  in  securing  the  motion  of  the  fluid.    The  blood  contains 
migratory  cells,  usually  colorless,  and  is  identical  with  the  fluid 
in  the  body  cavity.     The  general  body  contractions  are  im- 
portant in  causing  motion  of  the  fluids.     It  should  be  remem- 
bered that  the  water  vascular  system  is  also  partly  circulatory 
in  function.     The  blood  vessels  of  the  various  classes  agree 
in  having  a  central  circular  portion  consisting  of  one  or  more 
rings,  with  radial  tubes  running  into  the  arms,  and  in  some 
instances  vessels  which  accompany  the  intestine.     The  vessels 
of  the  oral  surface  are,  throughout,  in  close  connection  with 
the  nervous  epithelium  (Fig.  96,  r.b>). 

244.  Excretion, — It    is    impossible   to    name    any    organs 
known  to  be  solely  excretory  in  function.     As  in  respiration 
many  organs  may  take  part  in  the  work.     The  gaseous  and 
soluble  excreta  are  eliminated  through  the  general  body  sur- 
face, the  papulae,  the  respiratory  tree,  or  the  ambulacral  organs. 
The  skeletal  ossicles  in  the  mesoderm  represent,  in  part  at 
least,  the  elimination  of  certain  inorganic  salts  which  can  not 
be  used  in  the  vital  activities,  and  are  therefore  excretions. 

245.  The  Muscular  System. — The  degree  of  the  develop- 
ment of  the  muscular  system  varies.     In  those  forms  which 
have  a  well-developed  skeleton  the  body  muscles  are  not  of 
much  significance.     In  the  holothurians,  on  the  contrary,  the 

15 


210 


ZOOLOGY. 


FIG.  98.  Diagram  of  a  portion  of  the  water-vascular  (ambulacral)  system  of  the 
Starfish,  a,  ampullae;  f,  ambulacral  feet;  m,  madreporic  body;  p,  Polian  vesicles;  r.c., 
ring  canal,  with  the  upper  portion  removed  at  the  right  of  the  figure;  r.t.,  radial 
water  lubes  (in  r.t/  the  upper  portion  is  removed  at  the  distal  end  and  the  proximal 
portion  is  represented  entire) ;  s,  stone  canal. 

Questions  on  the  figure. — Where  does  the  water  enter  this  system 
of  vessels?  At  what  points  in  the  system  is  it  of  use?  By  comparing  all 
illustrations  at  your  disposal,  describe  the  mode  of  using  this  system  of 
organs  for  locomotion.  How  may  it  be  used  in  respiration? 

body  is  capable  of  definite  and  considerable  contractions,  by 
reason  of  both  circular  and  longitudinal  fibres.  In  forms 
with  incomplete  skeletons,  as  the  star-fish,  muscular  fibres 
connect  the  ossicles  and  there  is  a  degree  of  flexion  of  the 
arms.  There  are  also  special  muscles  controlling  the  water 
vascular  system,  the  stomach,  the  mouth  parts,  the  spines  and 
pedicellarise.  The  fibres  are  non-striate. 

246.  The  Nervous  System  consists  of  a  ring  around  the 
mouth  and  a  radial  nervous  band  in  each  arm  supplying,  by  a 


ECHINODERMATA.  211 

plexus  of  fibres  and  cells,  all  the  radial  organs.  This  system  is 
superficial  ("  ventral  ")  to  the  radial  water-tube  (Fig.  97,  r.  n.) 
and  in  the  star-fish  preserves  its  connection  with  the  ectoderm 
from  which  it  is  in  all  forms  derived.  Other  deeper  lying, 
and  even  aboral,  nervous  elements  are  described  for  some  of 
the  members  of  the  group.  These  elements,  when  present, 
have  as  their  function  the  innervation  of  the  muscles  of  the 
interior  and  of  the  aboral  wall  of  the  body. 

Sensory  organs  are  not  highly  developed.  The  animals 
show  evidences  of  possessing  a  chemical  sense  (analogous  to 
taste  and  smell)  by  which  the  presence  of  food  is  detected. 
This  is  apparently  localized  in  the  tentacles  in  such  forms 
as  have  them.  A  tactile  sense  is  also  present,  and  is  most 
highly  developed  in  the  tentacles,  ambulacral  feet,  and  other 
movable  outgrowths.  At  the  tip  of  the  antimeres  of  the 
asteroids,  or  of  the  radial  nerve  (echinoid)  are  structures 
bearing  pigmented  spots,  which  appear  to  be  sensitive  to  light 
(eye-spots).  These  cannot  give  more  than  a  very  general 
impression  of  light,  by  means  of  the  chemical  changes  induced 
in  the  pigment  cells  by  the  action  of  the  light, 

247.  Reproduction  is  wholly  sexual.     The  sexes  are  dis- 
tinct, but  the  males  and  females  are  not  often  distinguishable 
by  external  characters.     The  sexual  organs,  ovaries  or  testes, 
are  lobed  bodies  occurring  usually  in  pairs  in  an  interradial 
position.     These  open  by  pores  also  interradial,  and  usually 
dorsal  (Fig.  97,  r.  p.).  There  are  typically  five  pairs  of  genital 
glands,  but  in  the  holothurians  the  number  is  reduced  to  one. 
Fertilization  takes  place  outside  the  body,  and  usually  the 
development  is  wholly  independent  of  the  parent.     In  some 
instances  however  the  parent  has  special  pouches  in  which 
development  proceeds.  £ 

248.  Development. — The  fertilized  ovum  undergoes  total 
and    practically    equal    segmentation,    producing    a    ciliated 
blastula.      Gastrulation   occurs  by   invagination   resulting  in 
ectoderm  and  entoderm.     The  mesoderm  is  formed  in  two 


212  ZOOLOGY. 

ways  :  ( i )  by  migrating  cells  budded  from  the  entoderm  into 
the  segmentation  cavity  (mesenchyma;  Fig.  12,  c)  ;  and  (2) 
by  the  outgrowth  of  ccelomic  vesicles  or  pouches  from  the  wall 
of  the  archenteron  or  entoderm  (true  mesoderm).  These 
latter  outpockets  of  the  wall  of  the  gut  are  those  which  give 
rise  to  the  coelom  and  to  the  water  vascular  system  (see 

§241). 

In  the  later  larval  development  the  cilia  of  the  gastrula 
become  limited  to  two  zones, — a  preoral  and  a  preanal, — 
and  the  shape  of  the  larva  is  much  modified.  Numerous 
paired,  lateral  outgrowths  serve  to  accentuate  the  funda- 
mental bilateral  symmetry.  In  most  members  of  the  group 
a  marked  metamorphosis  occurs  in  the  passage  from  the  larval 
to  the  adult  condition.  During  this  change,  the  water  vascular 
system  and  the  mid-gut  of  the  larva  are  retained  with  the 
necessary  modifications.  About  these  as  a  centre,  what  we 
might  almost  call  a  new  animal,  the  radiate  star-fish,  begins 
to  grow  at  the  expense  of  the  larval  organs  which  are  ab- 
sorbed by  the  amoeboid  cells,  and  thus  new  organs  appropriate 
to  the  adult  are  formed.  During  this  process  the  bilateral 
symmetry  of  the  embryo  gives  place  to  the  radial  symmetry 
of  the  adult.  While  there  is  no  reproduction  by  budding 
there  is  a  striking  power  of  renewal  of  arms  or  other  portions 
which  may  be  lost  by  injury,  or  in  some  instances  by  self- 
mutilation.  Arms  are  readily  reproduced  if  the  disc  is  un- 
injured (stars,  brittle-stars,  and  crinoids)  ;  portions  of  the 
internal  organs,  as  the  digestive  tract,  are  said  to  be  regen- 
erated by  some  of  the  holothurians.  Occasionally,  at  least, 
an  arm  seems  to  have  the  power  of  reproducing  a  new  disc 
and  other  arms.  This  power  of  throwing  off  arms  and  re- 
placing them  is  doubtless  a  means  of  defense. 

249.  Ecology. — The  echinoderms  are  marine.  The  larvae 
are  free-swimming, — pelagic, — but  after  the  assumption  of 
the  adult  form  they  usually  become  much  less  active.  The 
crinoids  are  typically  stalked  and  often  attached.  The 
asteroids  and  echinoids  inhabit  the  bottom  of  the  ocean  where 


ECHINODERMATA.  213 

they  creep  more  or  less  slowly.  They  may  be  found  at  almost 
any  depth,  from  the  shallow  pools  at  low  tide  to  the  deepest 
bottoms.  Many  of  them  burrow  in  the  mud  and  sand,  and 
others  (some  sea-urchins)  have  the  power  of  scouring  out 
burrows  in  the  rocks  by  the  action  of  their  spines.  Echino- 
derms,  being  slow  movers,  are  compelled  to  subsist  upon  such 
food  as  the  currents  or  the  chance  movements  of  other  ani- 
mals may  bring,  or  upon  the  debris  which  falls  to  the  bottom 
of  the  sea,  or  upon  such  organisms  as  are  attached  and  cannot 
escape.  The  star-fishes  for  example  are  a  constant  menace 
to  the  oyster  beds.  The  fact  that  some  star-fish  are  in  a  meas- 
ure gregarious  makes  this  all  the  more  true.  It  is  difficult  to 
see  how  the  star-fish  can  get  the  oyster  from  the  protection  of 
its  shell ;  but  it  manages  to  get  the  shell  open  and  clasping  its 
arms  about  its  prey  it  turns  the  cardiac  portion  of  its  stomach 
inside-out  over  the  soft  part  of  the  oyster  and  thus  leisurely 
digests  it  outside  its  body,  so  to  speak,  leaving  the  empty  shell 
behind.  Except  for  this  the  group  is  of  little  economic  im- 
portance. The  Chinese  esteem  some  species  of  Holothuria 
(the  trepang,  for  example)  as  food.  The  group  appeared 
early  in  geological  time  and  has  had  very  characteristic  rep- 
resentatives in  all  ages  up  to  the  present.  The  changes  which 
have  taken  place  in  the  echinoderms  from  one  geological  age 
to  another  are  among  the  most  interesting  and  instructive 
furnished  by  the  invertebrates. 

250.  Classification. — Class  I.,  Blastoidea;  Class  II.,  Cystoidea. 

(These  are  both  extinct,  fossil  classes.  They  comprise  stalked  and 
attached  forms,  and  perhaps  represent  the  nearest  approach  of  our  known 
species  to  the  primitive  echinoderms.) 

Class  III.  Crinoidea  (feather-stars  and  sea-lilies}. — These  forms 
are  less  common  than  in  earlier  geological  times,  when  they  must  have 
been  very  abundant  and  very  beautiful.  They  contribute  much  to  the 
formation  of  the  limestone  of  the  Palaeozoic.  They  are  usually  provided 
with  jointed  stalks,  by  which  they  may  be  attached  to  the  bottom.  At  the 
summit  of  the  stalk  is  a  central  disc  with  five  arms  often  much  branched 
and  bearing  lateral  pinnules.  The  anus  is  on  the  oral  or  upper  surface, 
the  stalk  arising  from  aboral  surface.  They  are  inhabitants  of  the  deep 
sea. 

Class  IV.  Asteroidea  (star-fishes}    (Fig.  95). — The  asteroids,  of  which 


214  ZOOLOGY. 

there  are  several  hundred  species,  are  free  echinoderms  with  a  central 
disc  and  usually  five  arms.  The  latter  are  large  and  contain  liberal 
coelomic  spaces  in  which  are  lodged  outgrowths  of  the  digestive  system 
and  other  organs.  There  is  a  distinct  oral  and  aboral  surface.  The  anus 
and  madreporic  body  are  on  the  latter.  Distinct  ambulacral  grooves  lie 
on  the  oral  surface  of  the  arms.  Adult  star-fish  may  vary  in  size  from 
a  few  inches  to  two  feet  or  more  in  diameter. 

Class  V.  Ophiuroidea  (brittle-stars}. — These  are  fragile,  free  echino- 
derms in  which  the  arms  are  small  and  much  more  distinct  from  the 
disc  than  in  the  asteroids.  The  organs  of  the  disc  are  not  continued 
into  the  arms.  There  is  no  anus,  no  ambulacral  grooves,  and  the  madre- 
poric body  is  on  the  oral  surface.  Their  slender  arms  are  useful  in  cling- 
ing to  supports  or  to  prey,  and  are  used  in  locomotion. 

Class  VI.  Echinoidea  (sea-urchins,  sand-dollars'). — These  are  free 
echinoderms  without  free  arms.  The  arms  are  connected  by  the  develop- 
ment of  interradial  plates.  The  calcareous  rods  are  united  into  plates 
which  produce  a  complete  external  skeleton  varying  from  flat  dome-shape 
•  (as  in  sand-dollars)  to  a  globular  form  (Echinus  or  Arbacia).  The 
mouth  is  usually  in  the  centre  of  the  oral  surface  and  the  anus  near  the 
centre  of  the  aboral,  yet  one  or  both  may  come  to  have  an  excentric  posi- 
tion. In  this  way  the  bilateral  symmetry  is  accentuated  at  the  expense  of 
the  underlying  radial  symmetry.  The  madreporic  body  is  aboral  and  there 
are  no  ambulacral  grooves.  The  spines  of  the  urchins  are  usually  well 
developed  and  may  be  used  to  scour  out  rounded  pockets  in  rock  in  which 
the  animals  are  sometimes  found. 

Class  VII.  Holothuroidea  (sea-cucumbers'). — These  are  soft,  free 
echinoderms,  elongated,  cylindrical  or  flat,  with  mouth  and  anus  at 
opposite  poles  of  the  horizontal  long  axis.  The  skeleton  is  not  well- 
developed,  usually  being  represented  merely  by  scattered  spicules.  The 
water-vascular  system  in  most  forms  communicates  with  the  body  cavity 
instead  of  the  exterior.  Well-developed  tentacles  occur  about  the  mouth. 
Most  holothurians  burrow  in  the  sand  or  mud,  but  others  cling  to  rocks 
near  the  surface  of  the  water,  and  still  others  occur  at  great  depths  in 
the  ocean. 

251.  Suggestive  Studies  for  the  Library  or  Laboratory. 

1.  Read  and  report  on  the  metamorphosis  of  the  various 
members  of  the  group. 

2.  Study  from  dry  and  moist  material  and  report  on  the 
structure  and  mode  of  action  of   "  Aristotle's  lantern "   in 
Echinus. 

3.  Construct  a  table  of  parallel  columns — one  for  each  of 
the  five  living  classes — and  contrast  them  as  to:  (i)  general 
form  of  body  including  symmetry,   (2)   manner  of  motion, 


ECHINODERMATA.  2 15 

(3)  position  of  mouth  and  anus,  (4)  position  of  madreporic 
body,  (5)  character  of  digestive  tract,  (6)  differences  in  the 
spines  and  other  skeletal  structures,  (7)  the  position  and 
character  of  the  ambulacral  feet,  (8)  habitat  and  food,  (9) 
parts  repeated  in  the  antimeres. 

4.  Report  on  the  habits,   appearance,   and   abundance   of 
crinoids  in  geological  time. 

5.  The    origin    and    development    of    the    water-vascular 
system. 

6.  Compare  the  figures  of  the  various  classes  as  illustrated 
in  your  reference  texts  and  mark  the  degree  of  variation. 

7.  What  evidences  can  you  find  for  the  statement  that  the 
ancestors  of  the  present  Echinoclerms  may  have  been  bilateral 
forms  ? 


CHAPTER    XV. 
PHYLUM  V.— ANNULATA  (SEGMENTED  WORMS). 

LABORATORY  EXERCISES. 

252.  The  Earthworm  (Allolobophora  or  Lumbricus). — 
The  principal  work  should  be  done  with  living  worms.  For 
whatever  anatomical  work  is  undertaken,  specimens  may  be 
killed  by  exposure  to  fumes  of  chloroform  while  wrapped  in 
cloth  moistened  with  water;  they  should  then  be  pinned  out 
straight,  and  hardened  in  an  abundance  of  alcohol.  If  needed 
in  the  winter  they  may  often  be  found  under  manure  heaps, 
or  about  green-houses.  They  may  be  kept  alive  in  flower  pots 
containing  moist  earth. 

1.  Promorphology;  General  Form. — Is  there  an  anterior 
and  a  posterior  end?    How  distinguished?    Is  there  any  dis- 
tinction of  dorsal  and  ventral  surfaces?   If  so,  what?   Is  there 
bilateral  symmetry?  What  external  evidences  of  segmentation 
do  you  find?  How  are  the  similar  units  (metameres  or  seg- 
ments)  arranged?    Compare  with  the  condition  in  the  star- 
fish.    Compare  the  metameres  of  different  parts  of  the  body, 
noting  differences.     Is  the  body  divisible  into  regions   (i.  e., 
groups  of  similar  metameres)  ?    Locate   (by  numbering  the 
segments)  all  such  regions.     How  many  segments  in  the  ani- 
mal?  To  what  extent  does  this  vary  in  different  specimens? 
Show  by  a  series  of  diagrams  the  shape  of  the  animal,  and 
the  shape  and  size  of  cross  sections  in  various  regions. 

2.  Activities. — Describe,    after    careful    observation,     the 
method  of  locomotion  in  the  earthworm.     Place  the  worm  on 
a  rough  board:  on  a  plate  of  glass.     What  is  the  difference? 
And  why?   Compare  the  various  parts  of  the  body  as  to  size, 
during  movement.     Cause  of  the  difference?    Can  each  end 
move  foremost?    What  seems  to  determine  which  end  shall 
protrude  as  the  result  of  the  muscular  contractions? 

216 


ANNULATA. 

Does  the  animal  respond  equally  to  contact  (with  pencil  or 
toothpick)  at  anterior,  posterior,  and  middle  parts  of  the 
body?  Devise  a  method  of  determining  whether  it  is  sensitive 
to  light.  Record  results. 

Place  moist  soil  and  dry  soil  side  by  side  on  a  board;  place 
the  worm  in  various  positions  to  test  his  preference.  Record 
results.  Place  a  piece  of  filter  paper  which  has  been  dipped 
in  acetic  acid  in  the  path  of  a  worm.  How  does  it  react?  Try 
similarly  a  sugar  solution;  a  salt  solution;  a  decoction  of  de- 
caying leaves.  Will  an  earthworm  pass  into  water?  Do  your 
experiments  bear  in  any  way  on  the  habits  of  the  earthworm 
in  nature  ?  Can  you  secure  any  evidence  as  to  the  food  of  the 
earthworm  ? 

3.  Special  External  Structures. — Locate  the  mouth,  the  pre- 
oral   lobe,   clitellum    (a   series   of   swollen   segments),   anus. 
Compare  preoral  lobe  with  other  segments.    With  a  lens  and 
by  drawing  the  worm  backward  between  the  fingers  discover 
the  setae  or  bristles.    Are  they  found  on  all  segments  ?   Num- 
ber and  position  of  the  groups  of  setae  in  each  segment?  What 
is  the  function  of  the  setae?    Proofs? 

4.  Internal  Anatomy. — Pin  out  a  worm,  which  has  been  hardened  in 
alcohol,  on  dissecting  board  or  pan,  and  carefully  remove  the  dorsal  wall 
from  the  anterior  half  of  the  body  by  making  lateral  incisions  with  sharp- 
pointed  scissors,  or  make  a  single  incision  along  the  back  a  little  to  one 
side  of  the  middle  line.    After  noting  the  cross  membranes  (dissepiments}, 
their  relation  to  the  rings  on  the  outside,  and  their  attachments,  cut  them 
so  the  body  wall  may  be  folded  back  and  pinned.     The  dissection  should 
proceed  under  fluid, — 50  per  cent,  alcohol,  for  example.     Make  all  the  out- 
line drawings  necessary  to  show  all  your  discoveries.     Notice  the  coelom. 
Is  it  completely  divided  by  the  dissepiments?     Are  the  chambers  of  equal 
size? 

(a)  Digestive  organs :  Beginning  at  the  anterior  end,  note  the  following 
regions : 

Pharynx,  a  pear-shaped  enlargement :  how  held  in  place  ?    In  what 

segments  is  it  situated? 

CEsophagus,  a  narrow  tube;  crop;  gizzard;  intestine. 
Determine   the   segments   in   which   each    region   occurs.     Does   the 
digestive   tract   show   any   signs   of   segmentation,   i.   e.,   in  corre- 
spondence with  the  external  rings? 


218 


ZOOLOGY. 


(b)  Circulatory  system:    A  living  or  newly-killed   specimen   is   some- 
what better  for  this.     Discover,  if  possible : 

Dorsal  vessel  (just  dorsal  to  the  digestive  tract). 

Ventral  vessel  (just  ventral  to  the  digestive  tract). 

Hearts,    transverse    vessels    connecting    the    longitudinal    vessels,    in 

segments  VII  to  XL 
There  are  other  vessels  more  difficult  to  find.     Examine  a  drop  of 

the  contents  of  the  blood  vessels  with  the  microscope. 

(c)  Reproductive   System:    These  organs   are  rather  too  complicated 
for  satisfactory  results  in  an  elementary  class.     Instead  of  a  detailed  ex- 
amination note  the  reproductive  segments  (in  the  region  of  the  oesophagus) 
with  the  whitish  bodies  showing  at  the  sides  of  the  alimentary  canal,  and 
ventral  to  it.    They  are  attached  to  the  septa.     (Compare  figures  in  various 
text-books.) 

(rf)  Nervous  System :    In  a  well-hardened  preparation,  identify : 
Brain,  two  whitish  ganglia  just  dorsal  to,  and  in  front  of  the  pharynx : 
Collar,  around  the  mouth,  connecting  the  brain  with  ventral  ganglia, 
the  first  of  a  double  longitudinal  chain  of  ganglia  which  give  off 
nerves  in  each  segment.     How  are  the  ganglia  of  the  ventral  chain 
related  to  the  dissepiments? 

(e)  Excretory  Organs:  Just  lateral  to  the  nerve-chain  the  student  may 
be  able  to  find  coiled  thread-like  structures  (nephridial  tubes')  in  nearly  all 
the  body  segments  (see  text,  §264).  How  many  in  each  segment? 
.  5.  Microscopic  Demonstration. — The  teacher  should  make  or  secure 
good  permanent  mounts  of  transverse  sections  of  the  earthworm,  by  means 
of  which  the  students  should  make  out  the  following  points.  (See  Fig.  101.) 
Cuticle,  or  outer  layer. 

Body-wall,  and  the  relation  of  the  circular  and  longitudinal  muscles. 
The  ventral  nerve-chain  in  position. 
The  dorsal  and  ventral  blood  vessels. 
The  wall  of  the  digestive  tract;  gland  cells,  typhlosole,  etc. 

253.  Dero  (or  other  minute  aquatic  Annelid). — Any  one  of 
these  fresh  water  worms  may  be  used  very  profitably  to  sup- 
plement the  students'  work  on  the  earthworm.  Mount  the 
living  worm,  being  careful  to  support  the  cover-glass.  Study 
with  low  power.  Compare  at  all  points  with  the  earthworm. 
Dero  may  usually  be  had  at  any  season  of  the  year  by  taking 
mud  and  organic  matter  from  the  bottoms  of  foul  brooks  or 
ponds  and  placing  it  in  vessels  in  the  laboratory.  The  worms 
will  usually  come  to  the  sides  of  the  vessels  where  they  may 
be  seen.  Owing  to  its  transparent  qualities,  such  a  form  will 
be  especially  valuable  in  giving  the  student  a  better  idea  of 


ANNULATA.  219 

the  performance  of  function  in  the  group.  What  evidences 
of  muscular  action  are  manifest?  How  is  locomotion  effected? 
Position  and  mode  of  action  of  setae?  Study  the  capture  of 
food;  how  is  its  progress  through  the  digestive  tract,  and  its 
elimination  therefrom  effected?  Do  you  discover  any  circu- 
lation of  the  blood  ?  Direction  of  flow?  Evidences?  How  ac- 
complished ?  Test  for  ability  to  receive  and  respond  to  stimuli 
of  different  sorts.  Where  are  new  segments  formed?  Dis- 
cover, if  possible,  instances  of  fission,  by  which  new  individuals 
are  formed. 

254.  The  Leech. — The  leech  may  be  studied  and  compared  with  the 
earthworm  as  to  its  external  features,  its  habits,  mode  of  locomotion,  and 
the  like.    If  large  specimens  can  be  had  some  members  of  the  class  might 
substitute  it  for  the  earthworm  and  the  results  of  the  studies  brought  into 
comparison. 

255.  Nereis. — If  specimens  of  Nereis  can  be  obtained  this  worm  should 
be  compared  with  the  earthworm.      (Even  two  or  three  good  specimens 
may  be  made  useful  from  year  to  year  as  demonstration  both  of  external 
and  internal  structure.) 

Note  especially: 

(a)  The   specialization  of  the   anterior   end;   proboscis,   mouth,   jaws, 
palps,  cirri,  eyes,  antennae. 

(b)  The  fleshy  supports  of  the  bristles,  parapodia. 

DESCRIPTIVE  TEXT. 

256.  The  Annulata  are  separated  from  the  unsegmented 
worms  by  the  possession  of  a  series  of  segments  or  metameres 
which  show  on  the  exterior  as  rings,  and  contain  similar  or 
homologous  organs  or  similar  portions  of  a  continuous  organ. 
There  is  also  a  more  uniform  development  of  the  coelom  than 
in  the  lower  worms.     They  differ  from  the  ccelenterata  and 
echinoderms  in  having  bilateral  rather  than  radial  symmetry  in 
the  adult  condition.     The  development  is  often  direct,  but  in 
many,  especially  the  marine  forms,  there  is  a  metamorphosis. 
The  larva  has  a  peculiar  balloon-shaped  form,  known  as  the 
trochosphere   (Fig.   104,  E),  similar  in  some  respects  to  the 
Rotifers. 

257.  General  Characters. 

i.  Body  elongated,  bilaterally  symmetrical  and  segmented. 


220 


ZOOLOGY. 


FIG.  99. 


2.  External   paired .  appendages    (setae,   bristles,    etc.)    not 
jointed. 

3.  There  is  usually  a  well-developed  body  cavity. 

4.  The  excretory  organs  are  typic- 
ally paired  nephridial  tubules,  one  pair 
in  each  segment,  connecting  the  body 
cavity  with  the  outside.  Certain 
highly  modified  pairs  of  these  serve 
as  outlets  for  the  reproductive  bodies. 

5..  The  nervous  system  consists  of 
(i)  a  supra-cesophageal  ganglion 
(brain),  and  (2)  a  circum-cesopha- 
geal  collar  or  connective  uniting  it 
with  (3)  a  ventral  chain  of  ganglia 
with  a  ganglion  in  each  segment. 

6.  Locomotion  is  primarily  effected 
by  means  of  the  contractions  of  the 
body  wall,  acting  on  body  fluids  in  the 
cavity  within.- 

FIG.  99.  Dero,  a  fresh-water  oligochaetous  annelid, 
in  optical  (frontal)  section.  Enlarged  30  times,  a, 
appendages;  br.,  brain;  d,  dissepiments;  i,  intestine; 
m,  mouth;  nph,  nephridium;  oe,  oesophagus;  p,  pavil- 
ion, lined  with  ciliated  entoderm;  ph.,  pharynx;  pr., 
processes  from  the  anal  segment;  z,  zone  immediately 
in  front  of  the  anal  segment  where  new  segments  are 
continually  being  formed;  zf,  the  zone  of  fission  or 
budding.  This  takes  place  in  the  middle  of  a  seg- 
ment. The  anterior  half-segment  of  z'  will  produce 
a  region  like  z  for  the  anterior  half  of  the  worm. 
The  posterior  half-segment  will  produce  a  head  and 
four  segments  like  those  which  contain  the  pharynx 
(1-4)  of  the  parent  worm. 

Questions  on  the  figure. — What  regions 
of  the  digestive  tract  are  sufficiently  differ- 
entiated to  deserve  notice?  What  is  the 
number  of  the  segment  in  which  fission  is 
taking  place?  What  structures  must  the 
anterior  half  of  this  segment  make?  The 
segment  behind  the  dividing  segment  becomes 
number  5  of  the  new  posterior  worm.  What 
structures  then  must  be  developed  from  the  , 
posterior  half  of  the  dividing  segment? 


ANNULATA. 


221 


7.  Development  may  be  either  direct  or  indirect. 

258.  General  Survey. — The  Annulata  though  conforming 
to  the  type  outlined  above  are  very  diverse  in  appearance, 
habits  and  internal  structure.  While  the  Chsetopoda, — the 
class  to  which  the  forms  studied  in  the  laboratory  belong, — 
are  taken  as  the  type,  the  leeches,  which  have  no  bristles  but 
possess  suckers,  are  undoubtedly  related,  as  is  shown  by  their 
.development.  The  Rotifers  and  other  forms  are  sometimes 
included  among  the  relatives  of  the  Annulata.  Metamerism 
in  animals  is  a  most  interesting  phenomenon  to  zoologists. 
This  group  is  the  first  in  which  we  have  found  true  metamerism. 
The  body  of  the  animals  is  more  or  less  constricted  on  the 


n.  c. 


n.  f. 


co. 


FIG.  TOO.  Longitudinal  section  of  anterior  end  of  Dero.  A,  sagittal  section;  B, 
frontal  section  to  show  anterior  portion  of  nervous  system,  b,  brain;  co.,  nervous 
collar  about  the  mouth;  c.v.,  contractile  blood  vessels  ("hearts");  d,  dissepiment; 
d.m.,  dermo-muscular  wall;  d.v.,  dorsal  blood  vessel;  m,  mouth;  n.c.,  nerve  cells;  n.f.t 
nerve  fibres;  np.,  nephridia;  p,  prostomium;  ph.,  pharynx;  s,  setae;  sn.,  segmental  nerves; 
v.g,  ventral  chain  of  ganglia;  v.-v.,  ventral  blood  vessel.  Only  a  portion  of  the  blood 
vascular  system  is  shown,  and  this  appears  unsectioned  in  the  figure. 

Questions  on  the  figure. — Compare  this  with  the  cross-section  of  Dero 
and  identify  the  parts.  How  do  the  four  anterior  segments  differ  from 
the  others  figured?  Does  the  ventral  nerve  cord  continue  the  whole  length 
of  such  an  animal  as  this  ?  Which  organs  may  be  described  as  segmental 
and  which  as  continuous  through  the  segments  ? 


222  ZOOLOGY. 

outside  into  rings — as  the  name  (Annulata)  implies.  The 
internal  organs  also  show  metamerism,  but  in  various  ways. 
These  organs  may  pass  directly  through,  with  slight  segmental 
modification,  as  the  digestive  tube  and  ventral  nerve  cord; 
they  may  be  repeated  independently  in  each  segment,  as  the 
setae  or  nephridial  tubules ;  or  they  may  be  represented  in  only 
one  or  a  limited  number  of  segments,  as  the  brain  or  the 
reproductive  bodies.  The  segments  are  not  therefore  exactly 
equivalent,  yet  the  agreement  between  successive  segments  is 
sufficient  to  merit  the  term  homonomous  (see  §  121).  The 
number  of  segments  varies  from  three  to  hundreds.  The  body 
is  from  four  or  five  to  many  times  as  long  as  broad,  and  is 
usually  cylindrical  or  flattened  dorso-ventrally. 

259.  The  dermo-muscular  sac  is  composed  of  the  integu- 
ment or  skin  and  the  muscular  layers  of  the  body  wall.  Being 
filled  with  the  body  fluids  it  is  a  very  important  instrument 
of  locomotion.  This  is  accomplished  by  the  alternate  con- 
tractions of  the  circular  and  longitudinal  fibres  with  which 
the  wall  is  supplied.  Externally  there  is  a  cuticula,  usually 
very  thin,  overlying  and  secreted  by  the  layer  of  epidermal 
cells.  Some  of  the  cells  of  the  epidermal  layer  are  glandular 
and  others  are  sensory.  The  setae  or  bristles  are  secretions  of 
the  epidermal  cells  and  lie  in  sacs  in  the  skin.  These  struc- 
tures vary  in  number  and  position  but  are  usually  paired, — 
two  or  four  groups  to  each  segment.  They  are  absent  in  the 
leeches.  Next  to  the  skin  is  a  layer  of  circular  muscle  fibres, 
and  within  these  are  the  longitudinal  bands  of  muscle  fibres. 
In  the  leeches  there  are  also  dorso-ventral  fibres.  Special 
groups  of  fibres  occur  in  connection  with  the  setae,  the  mouth 
parts,  suckers,  etc.  The  fibres  in  worms  are  spindle-shaped 
and  unstriate.  The  dermo-muscular  wall  bounds  a  true  body 
cavity  in  the  chaetopods;  but  in  Beeches  the  ccelom  is  almost 
filled  with  connective  tissue.  This  suggests  the  condition  in 
many  of  the  unsegmented  worms.  See  Figs.  99,  101. 


ANNULATA. 
FIG.   101. 


223 


H.V., 


FIG.  101.  Transverse  section  of  Dero.  x  300.  c.,  coelom;  c.l.,  cells  of  the  so-called 
"lateral  line";  d.m.,  dermo-muscular  wall  including  muscles  and  skin;  d.v.,  dorsal 
blood  vessel;  ect,  ectoderm;  ent,  entoderm;  g,  gut;  g.f.,  giant  nerve  fibres;  gl,  glandular 
cells  assisting  in  digestion;  m.c.,  circular  muscle  fibres;  m.l.,  longitudinal  muscle  fibres; 
n,  nephridium;  n.v.,  ventral  nerve  chain,  made  up  of  nerve  cells  and  nerve  fibres;  s, 
setae;  v.v.,  ventral  blood  vessel. 

Questions  on  the  figure. — Compare  this  with  Fig.  100  and  identify 
all  the  structures  which  appear  in  both.  What  elements  enter  into  the 
dermo-muscular  wall  ?  Identify  nerve  cell's,  fibres  and  the  "  giant  fibres  " 
in  the  ventral  nerve  cord. 

260.  Worms  as  a  rule  have  no  external  skeleton  other  than 
the  cuticle,  but  in  some  instances  a  tubular  protective  structure 
is  formed  by  secretion  or  by  cementing  tog-ether  small  particles 
of  foreign  matter.     Because  of  the  absence  of  hard  skeletal 
parts  little  is  known  concerning  the  worms  of  past  geological 
ages. 

261.  Digestive  System. — The  stomodseum,  the  mesenteron, 
and  proctodseum  (see  §  90)  are  all  to  be  distinguished  in  the 
digestive  canal.    The  mouth  is  not  quite  terminal,  but  slightly 
ventral.     The  prostomium  (615  preoral  lobe),  a  muscular  ex- 
tension of  the  oral  segment,  overarches  it.     There  is  typically 
an  enlarged  muscular  pharynx  which  is  often  eversible,  fol- 
lowed by  a  narrow  tubular  oesophagus.     Often  there  is  no 


224 


ZOOLOGY. 


further  differentiation,  the  remainder  of  the  tube  being  fairly 
uniform  and  called  the  intestine.  Frequently  however  special 
enlargement?  occur,  chief  among  which  is  the  stomach.  In 
the  leeches  the  alimentary  system  is  much  modified  in  accord- 
ance with  the  blood-sucking  habit  of  the  animal.  The  crop 
is  capable  of  great  enlargement  and  may  contain  enough 
blood  to  nourish  the  animal  for  a  long  time.  The  mouth  is 
sometimes  armed  with  special  cuticular  outgrowths  which 
serve  as  teeth.  Glands  either  unicellular  or  compound  occur 
in  various  regions  of  the  digestive  tract.  In  the  earthworm 

FIG.  102. 


FIG.  102.  Transverse  section  of  the  intestine  of  the  Earthworm,  ty,  typhlosole,  an 
infolded  longitudinal  ridge  in  the  gut  in  which  the  gland  cells  (,gl.~)  are  especially 
aggregated.  Other  letters  as  in  Fig.  101. 

Questions  on  the  figure. — Of  what  conceivable  gain  is  the  typhlosole? 
What  is  it  analogous  to  in  the  higher  types  of  animals? 

•  *  ^IM— -t^^^MBBBlfrBB 

and  related  forms  there  is  a  dorsal  longitudinal  fold  of  the 
intestinal  wall  into  the  lumen  of  the  tube,  thus  increasing  the 
exposed  surface.  This  is  called  the  typhlosole  (Fig.  102)  and 
>plied  with  cells  which  have  been  described  as  digestive. 
The  enttfdermal  epithelium  may  secrete  a  cuticle  or  may  be 
ciliated.  This  layer  is  surrounded  by  a  connective  tissue  and 
muscular  fibres. 

262.  Respiration  is  effected  for  the  most  part  through  the 
general  body  wall,   into   which  the  blood  capillaries  or  the 


ANNULATA.  225 

lacunae  of  the  ccelom  may  penetrate.  In  some  forms  there 
are  special  thin  places  and  outpocketings  of  the  body- wall 
(branchiae)  by  which  the  exchange  of  gases  is  facilitated. 
These  are  characteristic  of  the  Polychaeta  especially  (Figs. 
105,  106). 

263.  Circulation. — In  some  of  the  simplest  worms  there 
are  no  special  blood  vessels.     The  ccelomic  spaces  contain  a 
fluid,  which  possesses  corpuscles  and  is  moved  by  the  general 
body  contractions.     In  the  typical  condition  there  are  two  or 
more  longitudinal  vessels,  dorsal  and  ventral  (or  lateral)  in 
position.     These  are  often  connected  by  transverse  loops  in  a 
few  or  many  segments  of  the  body  especially  at  the  anterior 
and  posterior  ends.     The  circum-intestinal   loops   are  often 
contractile,  and  the  longitudinal  vessels  may  show  a  wave  of 
contraction  passing  from  one  end  to  the  other.     Capillaries 
vary  much  in  perfection  of  development. 

264.  Excretion  takes  place  by  means   of  the  segmental 
organs  or  nephridia,  of  which  there  is  usually  one  pair  in  each 
segment,  with  the  exception  of  some  of  the  anterior  segments. 
The  nephridium  is  a  tubular  structure  consisting  essentially  of 
the  following  portions  (Fig.  33)  :  (i)  a  ciliated  funnel,  com- 
municating with  the  ccelom;  (2)  a  tortuous  glandular  region; 
and  (3)  an  outlet  through  the  body  wall,  often  supplied  with 
muscle  fibres.    The  nitrogenous  waste  products  find  their  way 
into  the  fluid  of  th£  ccelom  and  thence  into  the  nephridium, 
or  directly  into  the  nephridium  from  blood  capillaries  which 
may  occur  in  its  walls,  and  thus  are  finally  eliminated  upon 
the  exterior  of  the  body. 

265.  Nervous   System. — The   "central."   nervous   system 
may  be  said  to  consist  of  three  portions :  ( i )  a  mid- ventral  line 
of  nerve  fibres,  and  nerve  cells  which  are  diffusely  scattered 
or  collected  in  ganglia,    (2)   a  brain  which  is  anterior  and 
dorsal  to  the  pharynx,   (3)   a  connective  or  collar  about  the 
pharynx  connecting  (i)  and  (2)   (Fig.  100).     The  brain  and 

16 


226  ZOOLOGY. 

ventral  cord  may  show  distinct  right  and  left  lobes  or  may 
be  completely  fused  into  a  median  mass.  From  the  brain, 
nerves  pass  to  the  head-parts.  From  each  of  the  segmental 
portions  of  the  ventral  chain  nerves  pass  to  the  walls,  viscera, 
etc.  The  ventral  cord  frequently  lies  in  a  blood  sinus  which 
secures  its  abundant  nourishment  (leeches). 

The  sense  organs  occur  very  unequally  in  the  group.  The 
Polychaeta  and  the  leeches  are  best  supplied.  The  skin  is 
generally  sensitive  to  contact  and  chemical  stimuli.  This  sensi- 
tiveness is  perhaps  specially  localized  in  the  tentacles,  cirri, 
and  more  movable  parts.  Otocysts,  fluid-filled  cavities 
bounded  by  sensory  epithelium,  occasionally  occur  (see  §  108). 
Some  solid  particles  (otoliths)  float  in  the  fluid.  These  have 
been  described  as  organs  of  hearing  but  the  sensation  resulting 
is  probably  quite  different  from  what  we  know  as  hearing. 
They  are  apparently  organs  of  equilibration,  enabling  the 
animal  to  appreciate  its  position  in  relation  to  the  pull  of 
gravity.  Eyes  may  consist  merely  of  a  group  of  pigmented 
cells  with  nervous  connections,  or  may  be  very  complicated, 
consisting  of  a  capsule  with  refractive  media  and  retina. 
Images  of  Objects  are  not  formed,  in  all  probability,  but  the 
direction  and  intensity  of  light  can  be  appreciated.  In  the 
leech  there  are  sense  organs  in  each  segment  somewhat  similar 
in  structure  to  the  eyes.  Their  function  is  unknown. 

266.  Reproductive  Organs. — The  Oligochaeta  and  the 
leeches  are  hermaphrodite.  In  the  Polychaeta  the  sexes  are 
separate.  The  sexual  products  are  developed  from  the  ccelomic 
epithelium,  sometimes  on  the  dissepiments,  sometimes  on  the 
body  wall,  or  in  other  special  regions.  The  elements  may  be 
produced  in  many  segments  (Polychaeta),  or  in  a  few  anterior 
ones  (Oligochaeta).  The  region  is  usually  distinguishable 
only  about  the  breeding  time.  In  the  ^ejroa^hrodite  forms 
the  ova  and  spermatozoa  often  mature  at  different  times  and 
are  produced  in  different  segments.  This  of  course  insures 
cross-fertilization.  In  the  Polychaeta  the  conditions  are  rela- 
tively simple.  The  elements  are  freed  in  the  body  cavity  and 


ANNULATA. 


227 


when  mature  find  their  way  into  the  water  where  fertilization 
takes  place.  The  organs  are  much  more  complicated  in  the 
hermaphrodite  worms.  The  spermatozoa  are  produced  in 
the  testes,  are  passed  into  the  seminal  vesicles  where  they 
are  matured,  and  at  the  time  of  copulation  escape  to  the 
exterior  by  the  vasa  deferentia,  to  be  deposited  in  the 
sperm  sacs  or  reccptacula  seminis  of  another  worm.  From 
this  place,  any  time  after  copulation,  the  sperm  is  brought  into 
contact  with  the  ova  as  they  pass  from  the  ovary,  where  they 
are  produced,  to  the  egg-sac  or  to  the  exterior;  or  the  sperm 
and  ova  may  be  brought  together  after  both  have  escaped  from 
the  body.  It  is  believed  that  in  some  instances  at  least  the 
genital  ducts  are  modified  nephridia. 

267.  Reproduction  and  Development. — Sexual  repro- 
duction is  universal.  As  we  have  seen,  copulation  may  occur 
or  the  elements  may  come  together  in  the  water.  In  the 
Oligochaeta  and  leeches  the  fertilized  ova,  or  the  ova  together 
with  masses  of  spermatozoa,  are  enclosed  in  a  cocoon  of 
secreted  material  and  within  this  case  the  young  worm  is  de- 
veloped. In  the  Polychseta  the  larva  undergoes  its  develop- 


FIG.  103.  Two  stages  in  the  development  of  Nereis.  A,  8-celled  stage;  B,  i6-celled 
stage,  both  viewed  from  the  active  or  ectodermal  pole,  mi.1,  mi.2,  and  mi.3,  the  first, 
second  and  third  sets  of  micromeres  separated  from  ma.,  the  macromeres;  s1,  first 
somatoblast,  one  of  the  second  group  of  four  cells  to  be  budded  from  the  macromeres; 
s2,  second  somatoblast,  one  of  the  third  group,  which  gives  rise  to  the  mesoderm.  The 
micromeres  are  ectodermal  and  the  macromeres  produce  the  entoderm.  (After  West- 
inghausen.) 


228 


ZOOLOGY. 


FIG.  104.  Diagrams  of  stages  in  the  metamorphosis  of  Polygordius,  a  primitive 
annelid.  Ectoderm  throughout  is  represented  as  nucleated  without  cell  boundaries;  the 
entoderm  has  the  cell-boundaries  shown,  and  the  mesoderm  is  diagonally  shaded.  A, 
gastrula;  B,  same  with  blastopore  closed;  C  and  D  represent  formation  of  stomodamm 
and  proctodaeum  from  ectoderm;  E,  Trochosphere  stage  showing  formation  of  segments 
in  the  posterior  portion;  F,  adult  (sagittal);  G,  adult  (transverse),  a,  archenteron; 
bp.,  blastopore;  br,  brain;  c,  ccelom;  d,  dorsal;  di,  dissepiments;  nt,  mesenteron;  pr., 
proctodaeum;  s.c.,  segmentation  cavity;  st,  stomodaeum;  v.n.,  ventral  nerve  chain;  z, 
zone  of  formation  of  nerve  segments.  After  Fraipont. 

Questions  on  the  figure.— Trace  the  behavior  of  ectoderm  and  ento- 
derm in  these  figures  and  determine  what  structures  each  seems  to  give 
rise  to.  What  is  a  Trochosphere?  Distinguish  between  somatic  (body) 
and  splanchnic  mesoderm.  (See  §  56.) 

ment  in  a  free  state.  Segmentation  in  Annulata  is  complete 
and  usually  unequal,  giving  rise  at  the  eight-celled  stage  to 
four  micromeres  and  four  macromeres  (Fig.  103).  The 
micromeres  produce  the  ectoderm;  directly  or  indirectly  the 
macromeres  give  rise  to  the  entoderm.  Early  in  the  cleavage 
"  primitive  mesoblasts  " — cells  which  produce  the  mesodermal 
structures, — are  separated  from  the  macromeres.  A  gastrula 


ANNULATA.  229 

is  formed  either  by  invagination  or  by  overgrowth.  In  the 
earthworm  (Oligochseta)  the  blastopore  of  the  gastrula  forms 
the  mouth  of  the  adult  worm.  In  Nereis  (Polychaeta)  the 
blastopore  closes  by  growth,  and  the  stomodseum  and  proc- 
todaeum  arise  by  ectodermic  invaginations  which  finally  be- 
come continuous  with  the  entoderm  of  the  archenteron  (Fig. 
104,  D,  of  Polygordlus).  A  ciliated,  free-swimming  larval 
stage  ensues, — known  as  a  trocho  sphere  (Fig.  104,  E).  The 
trochosphere  may  be  looked  upon  as  representing  the  anterior 
or  head  end  of  the  adult.  The  later  metamorphosis  to  the  adult 
condition  involves  the  reduction  in  size  of  the  enormous 
anterior  region,  and  the  growth  of  segments  at  the  posterior 
end,  and  is  characteristic  of  Polychaeta.  The  development  of 
leeches  is  direct  as  in  the  Oligochaeta,  or  in  some  instances  it 
might  be  more  accurate  to  say  that  the  process  of  metamor- 
phosis is  very  much  abbreviated,  being  completed  by  the  time 
of  hatching. 

268.  In  addition  to  sexual  reproduction  many  worms,  par- 
ticularly the  aquatic  forms,  have  the  power  of  multiplying 
by  fission.  In  some  instances  this  may  consist  of  a  mere 
breaking  in  two,  as  was  seen  to  be  possible  in  the  star-fish, — 
each  part  regenerating  segments  corresponding  to  those  lost. 
In  other  cases  (Nais,  Dero,  etc.)  zones  of  rapidly  forming 
segments  are  produced  somewhere  in  the  mid-region  of*  the 
body,  and  from  this  zone  a  new  head  is  developed  for  the 
posterior  zooid  and  a  new  tail  for  the  anterior  zooid,  which 
usually  become  structurally  complete  before  the  separation 
takes  place  (Fig.  99,  #'). 

In  some  of  the  Polychaeta  (as  Autolytus)  a  distinct  alter- 
nation of  generation  is  found  in  which  sexual  and  non-sexual 
individuals  are  of  very  different  appearance. 

When  artificially  mutilated  the  earthworm,  and  some  other 
types  as  well,  may  regenerate  the  lost  portions.  Groups  of 
segments  of  one  worm  may  be  grafted  upon  another,  complete 
healing  taking  place  in  such  a  way  as  to  produce  an  apparently 
normal  worm.  Pieces  may  be  grafted  on  the  side  of  another 


230  ZOOLOGY. 

worm  in  such  a  way  as  to  produce  a   forked  or  otherwise 
abnormal  result. 

269.  Ecology. — The    leeches    are    aquatic    in    habit    and 
many  of  them  live  on  the  blood  of  higher  animals, — a  kind 
of  temporary  parasitism;  the  Polychaeta  are  marine,  and  the 
Oligochaeta  are  chiefly  fresh-water  or  terrestrial  in  habit.     A 
few  of  the  latter  groups  are  parasitic.     Of  the  aquatic  worms 
some  are  actively   free-swimming,   others  crawl  in  and  out 
among  the  living  and  dead  matter  of  the  bottom,  others  bur- 
row in  the  sand,  or  secrete  a  tubular  skeleton  into  which  they 
may  retire.    Their  chief  economic  importance  is  that  they  serve 
as  food  for  fish  and  other  food-animals.     The  earthworm,  in 
forming  its  underground  burrows,  eats  its  way  into  the  earth, 
swallowing  the  soil  for  the  organic  matter  which  it  contains 
and  passing  it  through  its  digestive  tract.     These  castings 
may  often  be  seen  at  the  mouth  of  the  burrows.    Worms  thus 
break  up  the  soil,  making  it  more  porous  and  accessible  to 
moisture,  bacteria,  and  the  rootlets  of  plants.     Darwin  esti- 
mates that  three  inches  of  the  subsoil  is  thus  brought  to  the 
surface  in  fifteen  years  through  this  agency. 

270.  Classification. 

Class  I.  Chcetopoda  (bristle-footed}. — Annulata  with  metameres  usu- 
ally well-marked  both  externally  and  internally;  with  setae  developed  from 
the  epidermis.  The  coelom  is  usually  voluminous  and  is  divided  into 
chambers  by  transverse  dissepiments.  Closed  blood-vascular  system. 
Ventral  nerve-chain  ordinarily  with  a  distinct  ganglion  to  each  segment. 

Sub-class  I.  Polychceta  (with  numerous  bristles'). — Marine  Chaetopoda 
with  numerous  setae  typically  borne  on  elevations  of  the  body  wall  (para- 
podia}.  Head  usually  well  differentiated,  bearing  eyes,  antennae,  cirri,  etc. 
Branchiae  or  gills  often  present.  Sexes  separate;  the  reproductive  organs 
simple,  and  repeated  in  many  segments.  A  metamorphosis  occurs;  the 
larva  is  known  as  a  trochosphere. 

Nereis,  the  "sand  worm"  of  fishermen  is  a  type  of  this  group. 
Autolytus  is  a  small  worm  especially  interesting  because  of  its  power  of 
reproducing  by  fission.  The  bud  which  is  freed  from  the  hinder  end  of 
the  worm  differs  from  the  parent  stock  in  that  it  is  sexual.  Amphitrite 
is  a  beautiful  worm  which  represents  the  attached  or  tube-forming  types. 
As  the  result  of  their  habits  such  forms  tend  to  lose  their  segmentation 
and  the  appendages  of  the  posterior  part  of  the  body.  The  gills  and 
tentacles  accumulate  about  the  head.  These  and  other  types  grow  abund- 


ANNULATA. 


23I 


antly  in  the  sand  and  mud  of  harbors,  amid  the  vegetation  of  the  bottom, 
and  over  exposed  objects  of  all  sorts  from  low  water  mark  to  unknown 
depths.  Their  value  in  utilizing  debris  and  the  more  minute  organisms 
as  food  and  thus  becoming  a  link  in  the  saving  of  these  to  serve  as  food 

FIG.  105. 


FIG.    105.     Amphitrite  ornata,  from  Verrill's  "  Invertebrate  Animals  of 
Vineyard  Sound." 


for  the  higher  organisms  cannot  be  over-estimated.  (Figures  105  and  106.) 
Subclass  II.  Oligochceta  (with  few  bristles'). — These  are  Chaetopoda 
with  no  parapodia  and  comparatively  few  setae  which  usually  occur  in 
two  or  four  clusters  in  each  segment.  They  are  mostly  fresh  water  or 
terrestrial  in  habit.  Fleshy  outgrowths,  such  as  gills,  are  almost  uni- 
versally absent.  The  sexes  are  united  in  one  individual  and  the  accessory 
reproductive  organs  are  very  complicated.  Ovaries  and  testes  limited 
to  a  small  number  of  anterior  segments;  development  direct.  The  head 
not  so  highly  specialized  as  in  the  Polychseta. 

The  earthworms,  of  which  there  are  numerous  species,  are  the  best 
known  types  of  this  subclass.  The  genera  and  species  are  distinguished 
chiefly  by  the  position  of  the  sexual  organs.  The  aquatic  OHgochaeta, 
which  are  much  smaller,  are  found  in  practically  all  ponds  and  ditches 
where  organic  matter  is  decaying.  The  more  common  genera  are  Dero 
(Fig.  99),  a  beautiful,  almost  transparent  worm  which  often  forms  a  tem- 
porary tube  for  itself  of  particles  cemented  by  its  own  secretion,  and 
Tubifex,  a  longer  worm  which  burrows  in  the  mud  at  the  bottom  of 
streams;  a  portion  of  the  body  protrudes  from  the  mud  and  waves  gently 


232 


ZOOLOGY. 


back  and  forth  in  the  water.  They  may  occur  so  thickly  that  thousands 
may  be  seen  in  the  space  of  a  few  feet.  When  their  home  is  jarred  they 
speedily  withdraw  from  sight.  A  colony  of  Tubifex  nearly  always  has 
associated  with  it  one  or  more  genera  of  smaller  worms,  as  Dero  or  Nais, 
a  species  similar  to  Dero  but  with  eye-spots.  Dero  has  an  interesting 
respiratory  apparatus  at  the  posterior  part  of  the  body  (Fig.  99,  /?.),  one 
of  the  few  instances  where  Oligochaeta  possess  such  organs. 

FIG.  1 06. 


FIG.   1 06.     Cirratulus  grandis,   from   Veirill. 

Questions  on  Figs.  105  and  106. — Are  these  Chsetopods?  What  are 
your  evidences?  What  is  the  nature  and  function  of  the  numerous  out- 
growths (branchial  cirri)  ?  In  what  respects  are  they  differently  arranged 
in  the  two  types?  Are  these  Oligochaeta  or  Polychaeta?  Your  reasons? 

Class  II.  Discophora  (bearing  suckers). — Annulata  in  which  there  are 
secondary  external  rings  which  tend  to  obscure  the  metameres,  inasmuch 
as  the  external  and  internal  segmentation  do  not  coincide.  There  are  no 
bristles.  The  body  cavity  is  much  reduced  by  the  growth  of  muscles 
and  connective  tissue.  The  remaining  spaces  contain  blood  and  are  in 
communication  with  the  vascular  system.  Two  sucking  discs  are  present 
and  are  powerful  organs  of  attachment.  The  anterior  sucker  embraces 
the  mouth ;  the  posterior  is  near  the  anus.  Sexes  are  united  in  one  indi- 
vidual; testes  numerous,  ovaries  a  single  pair.  Development  direct. 
Marine,  fresh  water,  terrestrial,  or  parasitic  in  habit. 


ANNULATA.  233 

271.  There  are  several  other  groups  of  annulates  of  considerable  inter- 
est   to    the   zoologist    which    it    seems    necessary   to   pass   by   with   mere 
mention. 

Class:  Archi-annelida ;  a  few  primitive  forms,  as  Polygordius  (Fig. 
104). 

Class:  Sipunculoidea  (Gephyrea).  With  traces  of  segmentation  in 
the  embryo,  but  not  in  the  adult. 

Class:   Chsetognatha  (arrow  worms). 

Some  authors  would  place  here  also  the  Rotifers  (see  §230). 

272.  Suggestive  Studies  for  Library  and  Laboratory. 

i.  Look  up  the  characteristics  of  the  Archi-annelida,  the  Gephyrea,  or 
Sagitta,  and  report  on  their  likenesses  to  the  types  studied. 
•    2.  On  what  grounds  might  the  rotifers  be  associated  with  the  annulata? 

3.  Compare  the  "  segments  "  in  cestodes  and  annulata. 

4.  In  the  Chsetopoda  which  sets  of  organs  pass  through  all  the  seg- 
ments, which  are  repeated  in  essentially  all,  and  which  are  limited  to  a 
few? 

5.  Examine  and  report  on  the  habits  of  the  earthworm.     (Study  in  its 
natural  haunts  or  in  box  of  moist  earth  in  laboratory.)     What  are  its 
haunts?    Method  and  rate  of  burrowing?    Does  it  avoid  water?    What  is 
its  food?    How  taken?    Does  the  animal  prefer  light  or  darkness? 

6.  If  near  the  sea-shore  select  other  forms  and  report  in  a  similar  way. 

7.  Investigate  parasitism  among  the  Annulata. 

8.  What  is  the  economic  value  of  the  earthworm?    Of  other  worms? 

9.  Make  a  study  from  the  text-books  of  the  reproductive  organs  in  any 
of  the  hermaphrodite  Oligochseta. 

10.  In  how  many  species  of  aquatic  Oligochseta  do  you  find  reproduc- 
tion by  fission?    In  what  particulars  does  the  process  seem  to  differ  in 
the  different  species? 

11.  Outline  the  life-history  of  Autolytus,  including  the  origin  of  the 


CHAPTER    XVI. 
PHYLUM  VI.— MOLLUSCA. 

LABORATORY  EXERCISES. 

273.  The  Clam  (My a)  or  Mussel  (Anodonta,  Unio). — 
Either  the  marine  or  the  fresh-water  type  will  serve.  The 
latter  are  to  be  found  in  almost  all  our  streams  and  small 
lakes.  They  may  be  obtained  with  a  long  handled  rake  from 
the  shore  or  from  a  boat.  They  often  occur  partly  buried  in 
the  sand  or  mud.  If  kept  in  water  they  may  be  transported 
to  the  laboratory  and  placed  in  a  tub  of  water  with  a  few 
inches  of  sand  at  the  bottom,  where  something  of  the  physi- 
ology may  be  studied  with  profit.  If  they  cannot  be  collected 
when  needed  for  study,  care  should  be  taken  to  supply  plenty 
of  the  preservative  fluid  in  which  they  are  kept. 

i.  The  Living  Animal. — What  facts  were  observed,  in  col- 
lecting the  material,  concerning  their  haunts,  their  abundance 
in  different  localities,  their  range  in  size,  etc.  ?  Are  there  any 
efforts  at  active  feeding,  as  far  as  you  have  seen  ?  Do  all  your 
specimens  belong  to  the  same  species? 

From  the  specimens  in  the  tub  make  out  the  following 
points : 

Has  the  animal  power  of  voluntary  motion?  If  so,  what  of 
its  rate,  manner,  the  position  of  the  animal  during  motion? 
How  is  the  animal  supported  in  this  position?  Determine  an- 
•terior  and  posterior  ends,  right  and  left  sides,  dorsal  and 
ventral  surfaces.  To  what  extent  can  the  soft  parts  protrude 
from  the  shell?  Note  briefly,  for  later  reference,  the  position 
of  all  visible  structures.  How  widely  does  the  shell  open 
during  life?  Note  the  trail.  With  a  pipette  place  a  drop  of 
some  colored  but  harmless  fluid  (carmine  in  water)  near  the 
fringes  of  the  posterior  end,  and  note  the  results.  Vary  by 
introducing  salt,  sugar,  and  acid  in  the  solution.  Devise 

234 


MOLLUSCA.  235 

experiments  to  test  whether  the  animal  shows  sensitiveness 
to  stimuli  of  various  sorts :  jars,  contacts,  currents  in  the 
water,  light,  warmth  and  cold. 

2.  General   Form. — Renew   your   observations   concerning 
the  symmetry  of  the  clam  by  careful  examination  of  the  ani- 
mal.    Determine  and  show  in  a  sketch  all  the  points  distin- 
guishing the  anterior  and  posterior  ends.     Are  the  right  and 
left  halves  symmetrical?   Use  a  pair  of  empty  shells  for  com- 
pleter  study. 

The  shell :  what  is  the  relation  between  the  valves  ?  How 
are  they  held  together?  Are  they  normally  open  or  closed? 
Give  your  evidences?  To  what  extent  may  the  shell  open 
without  violence?  How  does  the  shell  vary  in  thickness  at 
various  parts?  Contrast  the  interior  and  exterior  as  to  finish 
and  markings.  Make  note  of  everything  found,  with  outline 
drawings,  showing  position.  Locate  the  following  regions 
and  structures: — hinge,  umbo  or  beak  (a  prominence  near  the 
hinge),  hinge  ligament,  hinge  teeth,  pallial  line  (a  slight  de- 
pression marking  the  attachment  of  the  mantle  muscle), 
muscle  impressions,  lines  of  growth.  Review  after  studying 
soft  parts. 

What  is  the  oldest  portion  of  the  shell?  Evidence.  How 
does  the  shell  grow?  How  did  the  internal  depressions  come 
to  be?  Evidence. 

What  layers  are  discoverable  in  a  broken  shell?  How  do 
the  inner,  outer,  and  middle  layers  differ  in  thickness  and 
appearance  ? 

Do  you  find  any  differences  worthy  of  note  in  different 
individuals  ? 

3.  Soft  Parts. — Remove  one  valve  (say  the  left)  by  cutting 
the  two  muscles  which  hold  the  valves  together.     Leave  all 
the  soft  parts  in  the  right  valve  as  little  disturbed  as  possible. 
Make  a  sketch  showing  the  relation  of  the  body  to  the  shell.    If 
there  is  any  difficulty  in  cutting  the  muscles,  the  clam  may  be 
made  to  open  by  immersing  it  in  water  heated  to  about  140°  F. 
Identify: 


236  ZOOLOGY. 

Left  mantle  flap.     How  related  to  the  right?   to  the  shell? 

Siphons :  modifications  of  the  posterior  margins  of  the 
mantle.  (These  will  be  conspicuous  or  rudimentary  in  ac- 
cordance with  the  species  studied.)  Number? 

Adductor  muscles  of  the  valves ;  number  and  position. 

Mantle  cavity.  Separate  the  right  and  left  mantle  lobes 
along  the  ventral  margin,  except  in  the  region  of  the  siphon, 
and  fold  back  the  left.  Where  is  it  attached  to  the  body? 
The  ventral  or  incurrent  siphon  opens  into  the  branchial 
chamber,  the  dorsal  or  excurrent  into  a  smaller  dorsal  cham- 
ber,— the  cloacal.  Verify  and  sketch. 

Gill  plates  or  sheets;  number  and  attachment.  Are  they 
symmetrical  on  the  two  sides?  The  eggs  and  developing  em- 
bryos may  be  found  in  the  outer  gill  cavity  at  favorable  times. 
(A  special  study  and  report  may  be  profitably  made  by  some 
student  on  the  structure  of  the  gills  as  shown  by  a  hand  lens 
and  the  low  power  of  the  microscope.  A  bit  of  the  living 
gill  from  a  fresh  specimen  should  be  examined.) 

Abdomen, — the  soft,  fleshy  mass  between  the  pairs  of  gills, 
which  terminates  in  a  more  solid  part, — 

Foot :  position  and  form  ? 

Mouth  and  labial  palps;  at  the  anterior  end  and  just  below 
the  adductor  muscle.  How  many  palps? 

(It  is  to  be  remembered  that  all  the  structures  examined 
thus  far  are  external  organs.  The  body  wall  has  not  been 
penetrated  at  all.  If  it  is  the  plan  to  study  the  anatomy  more 
closely,  the  following  are  the  chief  sets  of  organs  deserving 
attention. ) 

4.  Other  systems  of  organs. 

Circulatory  system.  Open  the  pericardial  cavity,  just  beneath  the  hinge 
and  a  little  posterior  thereto,  find  the 

Heart:    auricles  and  ventricle.     In  a  fresh  preparation  the  contrac- 
tions of  the  heart  may  be  observed. 
Vessels:    one  passes  in  each  direction,  but  they  are  not  easily  seen 

without  injecting. 

The  intestine  passes  through  the  ventricle  without  open  communica- 
tion with  it. 


MOLLUSCA.  237 

Excretory  organ. 

Organ    of   Bojanus,    or    kidney,    lies    just    beneath    the    floor    of   the 

pericardial  cavity,  one  part  on  either  side. 

Each  portion  is  a  dark-colored  sac,  with  an  abundant  blood  supply. 
Nervous  system.     (Traced  best  in  hardened  preparations.) 
Visceral  ganglia.    Look  between  the  gills  in  the  posterior  portion  of 
the  body,  beneath  the  posterior  adductor  muscle,  and  in  the  floor 
of  the  cloacal  cavity.     Number,  and  closeness  of  connection?    By 
careful    dissection   determine   what   nerves    leave   them.     Trace    a 
pair  of  these  forward  to  the 

Cerebral  ganglia,  on  either  side  the  mouth.     Note  the  connections 
between  the  cerebral  ganglia.     Trace  from  these  ganglia  the  con- 
nectives which  pass  ventrally  to  the 
Pedal   ganglia   in   the   muscular    foot,   close   to   its   union   with   the 

abdomen. 
Make  a  clear  diagram  showing  the  relations  of  these  three  pairs  of 

ganglia. 

Digestive  system. 

Begin  with  the  intestine  at  the  heart.  Trace  posteriorly  to  the  anus. 
What  is  its  relation  to  the  posterior  adductor  muscle?  Pass  a 
bristle  into  the  intestine  anteriorly  and  use  it  to  guide  the  dissec- 
tion. Trace  the  intestine  through  the  abdominal  mass,  and  plot  its 
course.  Identify  the  stomach,  the  oesophagus,  and  the  mouth.  The 
liver  is  a  brownish  or  greenish  mass  surrounding  the  stomach. 
Much  of  the  visceral  mass  through  which  the  intestine  coils  is  made 
up  of  the  large  reproductive  glands  which  open  into  the  mantle 
cavity. 

5.  Cross  Sections. — A  series  of  cross-sections  may  be  made  by  the 
teacher,  numbered,  and  used  with  profit  as  demonstrations.  For  such 
sections  the  soft  parts  of  the  animal  should  be  hardened  for  24  hours  in 
i  per  cent,  chromic  acid ;  then  one  day  each  in  70  per  cent,  and  90  per  cent, 
alcohol.  Keep  in  95  per  cent,  alcohol  for  a  few  weeks.  Cut  one  fourth 
to  one  third  inch  thick  and  number  so  as  to  be  able  to  locate  position 
of  section.  Float  in  dish  of  alcohol  and  identify  the  parts  found.  Make 
sketches  of  sections  passing  (i)  through  the  stomach,  (2)  through  the 
heart,  and  (3)  through  the  middle  of  the  posterior  adductor  muscle.  In 
the  absence  of  these  the  student  should  be  encouraged  to  make  a  diagram 
of  an  imaginary  cross-section  through  the  middle  of  the  body.  Include 
the  shell. 

274.  The  Oyster. — One  or  two  students  should  be  asked  to  prepare  a 
report  on  the  structure  of  the  oyster  and  present  to  the  class  an  account 
of  the  chief  points  of  contrast  between  the  oyster  and  the  clam.    The  adult 
oyster  is  fixed  by  one  of  its  valves.    Is  it  the  same  one  in  all  specimens  ? 

275.  The  Pond  Snail  (Liinn&a). 

i.  The  Living  Animal. — Observe,  both  in  its  natural  home 
and  in  glass  vessel  containing  water  in  the  laboratory. 


238  ZOOLOGY. 

To  what  does  the  animal  adhere  in  the  water  ?  Must  it  have 
solid  support?  Can  it  swim?  What  is  its  method  of  locomo- 
tion? What  does  it  eat,  and  how?  Can  you  determine  whether 
it  uses  the  air  in  breathing  or  gets  its  oxygen  from  the  water? 
Proof?  How  is  the  gliding  motion  effected?  Watch,  with  a 
lens,  one  crawling  along  the  side  of  the  glass  vessel.  Record 
signs  of  sensitiveness  to  stimuli,  by  experiments  of  your  own 
devising. 

2.  General  Form. — Is  there  any  sign  of  bilateral  symmetry? 
In  what  parts  ?  How  are  anterior  and  posterior  distinguished  ? 
Relation  of  the  shell  to  the  animal?     Identify: 

Head:  tentacles,  number  and  position;  eyes,  number  and 
position. 

Foot,  the  muscular  expansion :  shape,  changes  in  form  and 
position. 

Mouth. 

Respiratory  orifice,  position.  Under  what  circumstances 
seen? 

3.  Shell  (secure  empty  ones). — Make  sketches  of  the  shell 
and  identify  the  structures  referred  to  in  the  following  terms : 
apex,    aperture,    lip,    spire,    whorl,  suture,    columella.      (See 
Fig.  i 08). 

How"  would  you  describe  the  direction  of  the  spiral?  How 
many  whorls  ?  Have  the  young  and  old  the  same  number  ?  Can 
you  detect  lines  of  growth? 

4.  Soft  Parts. — These  may  be  removed  by  dropping  the  animal  sud- 
denly into  hot  water,  and  then  gradually  twisting  the  soft  portion  from 
the  shell.     It  will  scarcely  repay  the  trouble  to  do  more  than  re-identify 
the  following  parts :  mouth,  respiratory  orifice,  mantle  and  mantle  chamber, 
and  collar  (a  portion  of  the  mantle).    The  spiral  is  occupied  by  che  diges- 
tive tract,  its  glands,  the  reproductive  bodies,  etc. 

5.  Development. — Examine  the  stems  of  plants   and  the  sides  of  the 
vessel  in  which  snails  have  been  kept  for  some  days  for  gelatinous  cap- 
sules of  eggs.     They  are  almost  transparent  and  the  eggs  may  be  easily 
located.     What  seems  to  be  the  value  of  the  gelatine?    Number  and  ar- 
rangement of  the  eggs?    What  is  the  shape  of  the  eggs?    Get  the  earliest 
stages  possible,  and  watch  day  by  day  at  short  intervals,  or  compare  cap- 
sules of  different  ages.     If  care  is  taken,   some  idea  of  the   early  seg- 
mentation stages  may  be  obtained.    Look  for  the  blastula :  are  the  cells  of 


MOLLUSCA.  239 

the  same  size?  Do  you  find  a  gastrula?  What  are  the  first  signs  you  find 
of  differentiation  of  parts?  Look  for  different  stages  of  the  later  develop- 
ment. It  will  not  be  profitable  to  try  to  follow  the  changes  in  detail. 

276.  A   very   valuable   laboratory   exercise   may   be   had  by   comparing 
large  numbers  of  shells  of  a  single  species,  found  under  varying  condi- 
tions.    Compare  as  to   shape,  markings,   etc.,   and  see  whether  there  are 
individuals  connecting  your  extreme  groups.     The  land  snail   (Helix,  Fig. 
119)  is  favorable  for  such  study. 

277.  The  Squid. — The  teacher  should  at  least  have  a  few  specimens  of 
the  Squid,  from  which  the  pupils  may  be  required  to  get  some  idea  of  the 
general  form.     Drawings  should  be  made,  showing  all  external  features. 

Note  particularly : — 

Head :  tentacles,  number,  comparative  length ;   suckers  on  the  inner 

surface,  arrangement  of  suckers. 
Eyes :  number,  size,  position. 

Olfactory  organs  opening  beneath  folds  of  s^in  behind  the  eyes. 
Neck. 

Body:  general  shape.     It  is  surrounded  by  the 
Mantle;  note  the  fin  expansions  at  the_j>ost£rior  end.     Where  are 

the  attachments  of  the  mantle  to  the  body? 
Siphon;  how  related  to  the  mantle  cavity? 

What  are  your  conclusions  as  to  the  symmetry  and  the  normal  position 
of  the  squid  ?  Do  you  find  anything  from  your  external  examination  which 
would  lead  you  to  class  it  with  the  clam  and  the  snail? 

DESCRIPTIVE  TEXT. 

278.  The  group  Mollusca  embraces   from   10,000  to  20,- 
ooo  living  species  among  which  there  are  very  great  differ- 
ences, as  illustrated  by  forms  as  unlike  as  slugs/snails,  oysters, 
clams,  devil-fishes,  and  squids.     With  the  exception  of  a  few 
they  are  sluggish  animals,  and  largely  aquatic  or  frequenters 
of  moist  places.     Some  are  well  protected  by  external  armor 
and  others  are  perfectly  naked.     The  typical  adult  mollusk 
is  clearly  marked  off  from  both  the  radiate  animals  such  as 
echinoderms  and  the  segmented  animals  such  as  the  Arthro- 
pods and  the  Annulata,  but  some  of  the  simpler  types  of 
mollusks,  and  the  larvae  of  certain  of  them  which  undergo  a 
metamorphosis,  strongly  suggest  that  they  may  be  related  to 
some  of  the  unsegmented  worms. 

279.  General  Characters, 
i.  Body    soft,    un^^  ^  , 
without  segmented  a 


240 


ZOOLOGY. 


2.  The  organ  of  locomotion  is  a  muscular  thickening  of  the 
body,  called  the  foot,  which  is  variously  modified. 

3.  A  thickened  dorsal   fold  of  the  body  wall,  called  the 
mantle,  is  usually  present.     This  encloses  a  space,  external  to 
the  body,  known  as  the  respiratory  chamber. 

4.  The  mantle  secretes  in  many  cases  a  calcareous  shell,  at 
first  single  and  symmetrical,  but  usually  becoming  either  spiral 
or  separated  into  a  right  and  left  valve. 

5..  The  central  nervous  system  usually  consists  of  three  sets 
of  ganglia:  (i)  the  cerebral  or  "  brain,"  above  the  mouth,  (2) 
the  pedal,  in  the  foot  and  connected  with  the  cerebral  by 
nerves,  and  (3)  the  visceral,  also  connected  with  the  brain 
by  a  pair  of  nerves  (Fig.  36). 

6.  Except  in  the  headless  forms  (Acalephs)  a  tooth-bear- 
ing ribbon,  the  odontophore,  is  found  in  the  mouth. 


^  ifll  of  a  Bivalve  Mollusk.   inner  surface,      ad. a.,   depression  showing  the 

e  anterior  adductor  muscle;  ad. p.,  posterior  adductor  muscle;  h,  hinge 

Attachments  of  the  ligaments;  p,  pallial  line,   marking  the  attachment  of 

-  !es;    s, .  the    pallial    sinus,    marking    the    attachment    of    the    retractor 

of  the  siphon;   u,  umbo  or  beak. 

Questions  on  the  figure.— Which  is  the  dorsal  and  which  the  ventral 
-hell?     Is 'this  the  right  or  left  valve3     What  is  the  effect 
ictioj;,,.     fh9  -:  ;    muscles?     What  is  the  value  of  the 

••""ill  of  the   snail   does  the 


MOLLUSC  A. 


24I 


280.  General  Survey. — The  more  commonly  known  forms 
are  easily  recognizable  by  the  hard  calcareous  shell  which 
protects  the  soft  unsegmented  body  within.  The  shell  may  be 
in  two  sub-equal  valves,  right  and  left,  or  may  be  in  one  piece, 
in  which  case  it  is  usually  coiled  or  spiral  (Fig.  108),  The 
bivalved  types  are  able  to  open  and  close  the  shell  aftfer  the 
manner  of  a  box,  and  the  soft  parts  are  further  capable  of 

FIG.  108. 


/an.. 


FIG.  108.  Helix.  A,  an  empty  shell  in  section  from  apex  to  base,  a,  apex  of  shell; 
an.,  anus;  ap.,  aperture  of  shell;  c,  columella  or  axis  of  shell;  e,  eyestalk;  /,  foot; 
/,  lip  of  shell;  m,  edge  of  mantle,  which  secretes  the  shell;  r.a.,  respiratory  aperture; 
s,  suture,  between  the  whorls;  t,  tentacles.  B,  the  relation  of  the  animal  to  the  shell 
when  extended. 

Questions  on  the  figures — What  suggestions  of  bilateral  symmetry 
are  shown  by  the  snail?  Where  does  growth  occur  in  the  shell?  What 
are  the, functions  of  the  tentacles?  What  is  the  function  of  the  edge  of 
the  mantle  called  the  "collar"  (w)  ? 

protrusion  from  the  partly  opened  shell.  This  latter  power 
is  much  more  pronounced  in  the  univalved  types  (r  g.,  snail). 
The  fundamental  bilateral  symmetry  is  obscured  in  the  more 
sluggish  forms,  but  is  very  decided  in  such  active  animals  as 
the  squid  and  some  of  the  bivalves. 

One  of  the  most  interesting  points  of  difference  among  the 
members  of  the  group  is  the  degree  of  development  of  the 
-7 


242  ZOOLOGY. 

"head."  In  the  bivalves  (lamellibranchs)  there  is  a  very 
slight  cephalization,  or  collection  of  special  organs  about  the 
anterior  end.  For  this  reason  they  are  often  called  Acalephs. 
In  the  gasteropods  (snails,  etc.)  and  cephalopods  (squid),  on 
the  other  hand,  the  head  is  well  developed  both  as  to  special 
mouth  parts  and  nervous  organs. 

The  forms  with  shells  are  somewhat  more  limited  in  size 
than  the  cephalopods,  which  furnish  the  largest  representatives 
of  the  phylum,  measuring  in  extreme  cases  20  to  40  feet  in 
the  reach  of  the  arms.  ^ 

The  calcareous  shell  insures  abundant  fossil  remains,  repre- 
sentatives being  found  in  various  geologic  formations  from 
the  beginning  of  the  Palaeozoic  era  to  the  present. 

281.  Integument  (skin). — This  consists  of  a  layer  of  epi- 
dermal cells,  covering  a  deeper  dermal  layer  derived  from  the 
mesoderm.     The  former  is  made  up  chiefly  of  the  supporting 
cells  and  the  simple  glandular  cells  which  secrete  mucus,  or 
lime,  or  pigment.     In  many  forms  a  large  portion  of  the  epi- 
thelium in  the  mantle  cavity    (as  the  inner  surface  of  the 
mantle  and  the  covering  of  the  gills  in  Lamellibranchs)   is 
ciliate.     The  dermis  is  a  complex  of  connective  tissue,  muscle 
fibres,  pigment  cells,  etc.     The  mantle  is  a  fold  of  the  skin 
which  is  very  characteristic  of  Mollusca.    It  grows  out  from 
the  dorsal  wall  of  the  body  and  encloses  a  space  known  as  the 
mantle  cavity.    It  is  usually  important  in  respiration,  and  con- 
tains the  shell-glands. 

282.  '"..ells  are  formed  in  all  the  classes  of  Aiollttoca,  al- 
though naked  forms  occur  in  several  of  them.     The  shell  is 
a  true  secretion  or  excretion,  deposited  by  the  epithelial  layer 
of  the  mantle.    It  consists  of  three  layers  :  (a)  a  thin  external 
layer  of  organic  material  known  as  canchiolin,  (b)  the  pris- 
matic layer,  embracing  the  greater  thickness  of  the  shell  and 
made  up  of  prisms  of  carbonate  of  lime  cemented  by  con- 
chiolin,  and  (c)  the  nacreous  or  pearly  layer  over  the  inner 
surface.    The  edge  of  the  mantle  secretes  the  first  and  second 
layers,  and  they  usually  show  lines  of  growth  parallel  with 


MOLLUSCA.  243 

the  edge  of  the  mantle;  the  pearly  layer  is  deposited  by  the 
whole  surface  of  the  mantle.  The  point  of  attachment  of  the 
muscles  presents  a  depression  in  this  layer  because  the  deposit 
has  been  interrupted  (see  pallial  line  and  muscle  scars,  Fig. 
107;  and  in  shell  of  clam). 

In  some  Cephalopods  there  is  an  internal  skeleton  in  part 
secreted  by  the  mantle  (cuttle  bone),  and  in  part  formed  of 
cartilage  (the  brain  case). 

283.  The  muscular  system  is  made  up  of  unstriped  muscle 
fibres,  which  usually  occur  in  more  or  less  prominent  bands 
or  muscles.     These  may  be  classified  as  follows:  (i)  shell  or 
skeletal  muscles,  which  embrace   (a)  adductors,  those  which 
draw  the  valves  together   (lamellibranchs),    (b)   rectractors, 
which  withdraw  the  whole  or  special  portions  of  the  animal 
into   the   shell    (lamellibranchs   and   gasteropods),    (c)    pro- 
tractors or  extensors,  which  enable  the  animal  partly  to  extend 
itself;  (2)  pallial  (mantle)  muscles,  best  developed  in  cephalo- 
pods;  (3)  the  foot,  which  is  a  mass  of  muscle  and  is  one  of 
the  most  characteristic  of  the  molluscan  organs;   and    (4) 
minor  muscles  controlling  the  radula  or  tongue,  the  other 
mouth  parts,  and  the  like. 

Locomotion  in  the  group  is  accomplished  chiefly  by  the  foot, 
in  its  various  modifications,  or  by  rhythmic  opening  and  shut- 
ting of  the  valves.  The  squid  has  a  fin-like  extension  of  the 
integument  which  is  an  efficient  organ  of  forward  motion. 
The  siphon  of  the  same  animal  is  regarded  as  a  modification 
bf  a  part  of  the  foot.  The  tentacles  about  the  mouth  are  also 
looked  upon  as  arising  from  the  antengr  part  of  the  foot, 
hence  the  name  Cephalopod,  which  mean?  "head-footed" 

284.  Digestive  Organs. — Mouth  and  anus  both  occur,  and 
are  usually  widely  separated.     In  the  coiled   forms    (as  the 
snail),  however,  by  the  looping  of  the  digestive  tract  they  are 
brought  close  together.     In  all  except  the  group  of  headless 
mollusks  (lamellibranchs)  the  mouth  is  supplied  with  a  radula, 
or  tooth-bearing  tongue.     This  lies  in  the  floor  of  the  mouth 
and,  as  it  is  worn  away  in  front,  is  renewed  from  behind  in 


244  ZOOLOGY. 

the  radula  sac  (Fig.  109).  It  rasps  small  particles  from  solids 
and  conveys  them  backward  into  the  oesophagus.  In  the 
gasteropods  there  is  a  plate  in  the  upper  jaw  against  which 
this  organ  works.  In  the  cephalopods  beak-like  jaws  occur 
suited  to  their  carnivorous  habit.  The  mouth  is  followed  by 


FIG.  109.  Diagram  of  mouth  of  snail,  showing  the  lingual  ribbon  (radula).  br, 
brain;  c,  buccal  cavity;  co.,  coelom;  g,  gullet;  /,  jaw,  against  which  the  radula  works; 
m,  mouth;  r.,  radula;  r.s.,  radula  sac,  in  which  the  radula  is  renewed  as  it  is  worn 
away  in  front. 

Questions  on  the  figure. — What  parts  go  to  make  up  the  "  odonto- 
phore"?  How  do  the  parts  act  in  biting? 

a  gullet,  which  may  communicate  at  once  with  the  stomach 
(lamellibranchs),  or  may  expand  into  a  crop  (gasteropods  and 
cephalopods).  The  stomach  is  well  marked  and  opens  into  the 
intestine  which  is  usually  long  enough  to  make  one  or  more 
coils  in  the  body  mass.  It  may  open  externally  (gasteropods) 
or  in  the  mantle  chamber  (cephalopods  and  lamellibranchs). 
Salivary  glands  pour  their  secretion  into  the  mouth  cavity  or 
into  the  gullet,  and  the  so-called  liver  connects  with  the 
stomach  or  intestine. 

285.  Respiration. — The  oxygen  may  be  derived  from  the 
water  (lamellibranchs,  cephalopods,  and  some  gasteropods) 
or  from  the  air  (pulmonate  gasteropods).  In  the  latter  a 
pulmonary  chamber  is  formed  by  the  mantle.  Blood  is  richly 
supplied  to  the  walls  of  this  sac  and  is  there  aerated  after  the 
manner  of  lungs.  In  the  water-breathing  forms  the  gills  are 
variously  constructed.  Lamellibranchs  possess  a  pair  of  "  gill- 


MOLLUSCA. 


plates  "  hanging  in  the  mantle  cavity  on  either  side  the  body. 
These  are  made  up  of  an  immense  number  of  ciliated  tubular 
filaments  which  intercommunicate  in  a  complicated  lattice- 
work. To  the  naked  eye  they  appear  as  thin  sheets  with 
striations  passing  from  the  dorsal  to  the  ventral  margin  (see 
dissection  of  clam).  The  walls  of  the  gills  contain  blood 
vessels,  and  the  water,  assisted  by  the  action  of  the  cilia,  circu- 
lates over  and  through  the  gills.  In  the  cephalopods  and 
aquatic  gasteropods  the  gills  occur  as  tufts  of  filaments,  which 
may  or  may  not  be  covered  by  the  mantle.  In  addition  to 
these  special  organs  the  mantle  and  the  soft  body  surface 


FIG.  no.  Diagram  showing  the  heart  and  general  course  of  the  circulation  in  the 
Lamellibranchs.  Only  a  short  section  is  shown,  a,  auricle  (right),  with  slit  to  ven- 
tricle; b,  the  body  (region  of  spaces,  lacunae,  capillaries);  g,  the  region  of  the  gills,  with 
capillaries;  k,  kidneys,  with  their  capillaries;  m,  the  mantle  and  capillaries;  v,  the 
ventricle  from  which  arteries  pass  forward  and  backward;  v.c.,  "vena  cava,"  in  which 
the  blood  collects  on  returning  from  the  tissues  of  the  body. 

Questions  on  the  figure. — Follow  by  the  arrows  and  letters  the  general 
course  of  the  blood  flow.  How  many  sets  of  capillaries  are  passed  by  the 
blood  which  goes  to  the  mantle?  By  that  which  goes  to  the  system,  be- 
fore returning  to  the  heart?  What  changes  take  place  in  the  blood  in  the 
capillaries  of  the  various  regions? 


246 


ZOOLOGY. 


assist  in  respiration.  (For  figures  of  the  gill  structure  in  the 
clam  see  Parker  and  Haswell's  Text-book  of  Zoology,  Vol.  I, 
Fig.  529-) 

286.  Circulation. — There  is  usually  a  well-developed  circu- 
lation of  the  blood,  but  a  portion  of  it  occurs  through  irregular 
spaces  devoid  of  proper  walls.  The  organs  consist  of  a  con- 
tractile heart  usually  with  undivided  ventricle  and  a  single 
auricle  (gasteropods),  or  one  pair  of  auricles  (lamellibranchs, 


tissues 


tissues 


FIG.   in.     Diagram  showing  the  general   course  of  the  circulation  in  mollusks.     Com- 
pare with  Fig.  no,  which  shows  the  organs  more  nearly  in  their  relative  position. 

Questions  on  the  figure. — Why  does  the  blood  which  passes  to  the 
mantle  not  need  to  pass  to  the  gills  before  returning  to  the  heart?  What 
happens  to  the  blood  in  each  of  the  regions  named  in  the  diagram? 

squid),  or  two  pairs  (Nautilus).  Definite  arteries  pass  both 
forward  and  backward  from  the  ventricle.  The  blood  passes 
from  the  ventricle  to  the  tissues  of  the  body,  whence  it  gathers 
into  venous  spaces  and  passes  into  the  kidneys  and  the  gills. 
From  the  gills  it  finds  its  way  to  the  auricles.  In  lamelli- 
branchs the  bloojd  which  goes  from  the  ventricle  to  the  mantle 
returns  directly  to  the  auricle.  In  some  Cephalopods  there 
are  branchial  hearts  near  the  gills  to  assist  the  return  of  the 
blood  to  the  heart.  The  accompanying  diagrams  (Figs,  no, 
m)  will  help  the  student  follow  the  main  facts  of  the  circula- 
tion. In  lamellibranchs  the  ventricle  often  surrounds  the  in- 


MOLLUSCA.  247 

testine.  The  corpuscles  are  colorless  and  amoeboid.  The 
plasma,  however,  quite  commonly  contains  a  bluish  pigment 
(hsemocyanin)  which  assists  respiration  in  somewhat  the  same 
way  as  the  haemoglobin  of  the  vertebrates. 

287.  Excretory     Organs. — In     mollusks     the     excretory 
organs  consist,  when  reduced  to  the  simplest  terms,  of  one  or 
more  nephridia  which  communicate  interiorly  with  the  peri- 
cardium or  principal  coelomic  space,  and  with  the  exterior  by 
way  of  a  tubular  ureter.     The  kidney  portion  of  the  tube  is 
much  modified,  has  glandular  walls  and  is  well  supplied  with 
blood  vessels.     It  lies  in  the  immediate  region  of  the  peri- 
cardial  chamber  in  most  cases. 

288.  Nervous  System. — The  nervous  system  of  mollusks 
is  usually  made  up  of  at  least  three  pairs  of  ganglia:  (a)  the 
"  brain  "  nrxfrfb^1  g-anprlia  dorsal  to  the  mouth  and  varying 
in  size  according  to  the  degree  of  development  of  the  head; 
(b)  connected  with  the  brain  by  a  pair  of  connectives  are  the 

Lai  ganglia  lying  ventral  to  the  mouth  and  innervating  the 
foot ; ~~(c)  the  pleqir^yisceral  ganglia  variously  situated  in  the 
different  groups  and  connected  with  the  brain  or  both  with 
the  brain  and  the  pedal  ganglia.  From  it  nerves  pass  to  the 
mantle,  and  to  the  posterior  organs.  In  gasteropods  and 
cephalopods  these  ganglia  are  much  closer  together  and  are 
collected  about  the  mouth.  Still  other  ganglia  are  often  asso- 
ciated with  them.  The  student  should  notice  how  this  collec- 
tion of  nervous  matter  accompanies  the  development  of 
"  head "  organs  in  the  better  developed  members  of  the 
phylum. 

289.  The  Organs  of  Special  Sense. — As  usual,  scattered 
sensory  cells  are  situated  in  the  exposed  epithelial  surfaces. 
These  give  rise  to  a  diffuse  sensitiveness  to  tactile  and  chemi- 
cal stimuli.     The  edges  of  the  mantle  and  the  tentacles  are 
especially  sensitive.      Patches   of  sensory  cells — osphradia — 
are  often  found  near  the  bases  of  the  gills,  which  probably 
have  a  value  in  testing  the  character  of  the  water  flowing  over 


248 


ZOOLOGY. 


them.  Still  other  patches  occur  about  the  lips.  Otocysts  (see 
§  1 08)  occur  in  all  the  groups.  Eyes  are  usually  found  and  are 
of  various  degrees  of  complexity.  They  are  simplest  in  the 
lamellibranchs  (Fig.  41),  and  when  found  at  all  in  this  group 
may  occur  in  great  numbers  along  the  mantle  edge.  In  the 
gasteropods  the  eyes  are  borne  on  the  ends  of  tentacles  and 
are  frequently  destroyed  by  accidents.  The  animals  have  the 
power  of  regenerating  the  tentacle, — eye  and  all.  This  mani- 
festly is  a  very  useful  adaptation.  The  eyes  of  cephalopods 
are  the  most  perfect  single  eyes  found  among  the  invertebrates. 

FIG.  112. 


FIG.  112.  Diagram  of  a  dissection  of  the  reproductive  organs  of  a  snail,  a.g.,  albu- 
men gland;  c.d.,  common  or  hermaphrodite  duct;  e.g.,  hermaphrodite  gland;  d.s.,  dart 
sac;  f,  flagellum;  g,  genital  aperture;  tn.g.,  mucous  glands;  o,  oviduct;  p,  penis;  r^s., 
receptaculum  seminis;  v.d.,  vas  deferens.  The  slit  from  the  genital  aperture  into  the 
oviduct  and  penis  shows  the  openings  of  the  dart  sac,  mucous  glands,  and  the  recep- 
taculum seminis.  (After  Pelseneer.) 

Questions  on  the  figure. — By  a  careful  study  of  the  figure  and  the 
text,  determine  the  functions  of  the  various  parts  of  the  system.  Does  self- 
fertilization  occur  in  a  form  like  this  ?  Evidences. 


MOLLUSCA.  249 

Though  originating  in  a  different  way,  it  is  strikingly  like 
the  vertebrate  eye. 

290.  Library  Reference. — Make  a  report  on  the  position  and  general 
structure  of  the  eyes  in  gasteropods,  cephalopods  and  lamellibranchs. 

291.  Reproduction  and  the  Genital  Organs. — Reproduc- 
tion is  always  sexual.     In  some  of  the  lamellibranchs  (e.  g.f 
oyster)   and  many  of  the  simpler  gasteropods,  including  the 
land  snails,  the  individuals  are  hermaphrodite.     The  sexes  are 
separate  in  the  cephalopods  and  in  most  of  the  lamellibranchs 
and  gasteropods.     The  organs  are  more  complicated  among 
the    hermaphrodite  gasteropods  than  elsewhere  in  the  group 
(see  diagram  reproductive  organs  of  snail,  Fig.  112).     The 
sexual  glands  are  usually  situated  in  the  visceral  mass  among 
the  coils  of  the  intestine.     The  ducts  ordinarily  open  into  the 
mantle  cavity  where  fertilization  may  occur.     The  eggs  after 
fertilization  are  often,  either  singly  or  in  masses,  surrounded 
by  a  gelatinous  secretion  (as  in  the  snail)  which  serves  as  a 
protection  from  drouth  and  as  a  means  of  attachment.     In 
lamellibranchs  the  young  are  not  infrequently  retained  in  the 
mantle  or  respiratory  chamber  until  partly  developed. 

292.  Development. — Segmentation  is  total  (lamellibranchs 
and  gasteropods)  or  partial  and  discoklal  (dibranch  cephalo- 
pods).   It  is  usually  unequal  in  the  lamellibranchs  and  gastero- 
pods, but  in  some  of  the  latter  it  is  equal  during  the  first  two 
divisions,  producing  four  equal  blastomeres.     Each  of  these 
divides  into  a  large  and  a  small  cell — macromere  and  micro- 
mere.     Still  other  micromeres  are  formed  at  the  expense  of 
the  macromeres,  and  these  by  continued  division  form  a  cap 
of  ectodermal  cells  (Fig.  113).     From  the  macromeres  arise 
ultimately  the  entoderm  and  mesoderm.     The  gastrula  may 
be  formed  either  by  invagination  of  the  large  cells  or  by  the 
overgrowth  of  the  micromeres,  depending  on  the  size  of  the 
segmentation   cavity   and   of   the   entodermal   cells.      In   the 
cephalopods,  owing  to  the  large  supply  of  food  substance  in 
the  ovum,  cleavage  is  confined  to  a  small  disc  at  the  active  pole. 


250 


ZOOLOGY. 


From  this  point  where  the  embryo  is  destined  to  be  developed, 
a  sheet  of  cells  gradually  extends  itself  by  growth  around  the 
yolk.  Thus  a  yolk-sac  is  formed  by  means  of  which  the  food 
is  used  in  the  further  development  of  the  embryo.  By  the  time 
the  embryo  is  hatched  the  yolk  is  exhausted.  Although  the 
yolk  does  not  segment  we  see  that  it  serves  its  purpose  in  the 

FIG.  113. 


mes.- — 


•::.-/—  ent. 


FIG.  113.  Diagram  of  early  segmentation  stages  in  a  Gasteropod.  A,  2-celled  stage; 
B,  4-celled;  C,  8-celled;  D,  later  stage,  in  section,  ect.,  ectoderm  cells  (micromeres) ; 
ent.,  entoderm  cells,  macromeres;  mes.,  mesoblasts,  early  put  aside, — before  gastrula- 
tion — to  form  the  mesoderm;  mi.,  micromeres;  ma.,  macromeres. 

Questions  on  the  figures. — What  causes  are  assigned  for  the  differ- 
ence in  the  size  of  the  cells  in  the  8-celled  stage?  In  what  other  ways  is 
mesoderm  formed  in  the  metazoa?  Which  cells  seem  to  divide  more 
rapidly,  the  micromeres  or  the  macromeres?  Compare  with  Annelid, 
Fig.  103. 

development  of  the  embryo.  The  later  development  is  typi- 
cally indirect,  i.  e.,  with  a  metamorphosis,  though  many  (as 
the  cephalopods)  develop  directly  into  the  adult  form.  A 
larval  stage  (trochosphere)  occurs,  suggesting  the  larvae  of 
the  Polychaeta.  This  is  followed  by  another  stage  (veliger) 
which  is  more  characteristic  of  the  Mollusks. 

293.  Library  Exercises. — Appoint  students  to  supplement  the  text  by 
making  short  reports  on  the  following  topics:  the  early  segmentation  of 
lamellibranchs  and  gasteropods ;  of  the  cephalopods ;  the  veliger  of 
mollusks;  the  formation  of  the  organs  in  cephalopods;  development  in  the 


MOLLUSCA.  251 

clam  or  mussel.     Illustrations  should  be  found  in  the  advanced  text-books 
and  presented  to  the  class. 

294.  Ecology. — The  bivalves  are  sedentary  or  sluggish  in 
their  manner  of  life;  the  motion  of  most  of  the  gasteropods 
is  slow  and  difficult.  In  conformity  with  their  limited  powers 
of  locomotion,  they  are  scavengers,  feeding  on  the  debris  and 
the  small  animals  and  plants  brought  to  them  by  the  water 
currents  (oysters,  mussels,  etc.),  or  are  largely  herbivorous 
(many  snails).  A  very  few  are  parasitic.  The  cephalopods 
are  much  more  active  and  are  carnivorous.  For  the  most  part 
the  sluggish  forms  are  well  protected  by  the  shells,  neverthe- 
less they  furnish  food  for  many  diverse  sorts  of  animals. 
Some  of  their  enemies  are  internal  parasites,  others  bore 
through  the  shells  and  thus  gain  access  to  vital  parts. 

The  animal  within  may  thwart  this  attack  of  its  enemies  by 
the  continued  secretion  of  mother-of-pearl  on  the  inner  sur- 
face at  the  threatened  point.  Some  animals  crush  the  shells, 
or  swallow  the  mollusk,  shell  and  all.  Star-fishes,  as  we  have 
seen,  are  especially  troublesome  to  the  oyster  beds. 

Many  of  the  bivalves  are  capable  of  still  further  protection 
because  of  their  elongated  siphons  which  enable  them  to 
burrow  deeply  in  the  mud  or  sand,  the  food  being  carried  in 
through  the  siphons  by  the  water  currents  (Fig.  1 14).  Several 
species  of  marine  bivalves  have  the  power  of  boring  into  wood 
or  even  stone.  This  serves  as  a  protection  to  them,  but  often 
results  in  the  complete  destruction  of  piles  and  other  structures 
placed  in  the  ocean  by  man. 

Many  of  the  mollusks  seem  more  or  less  gregarious,  as  is 
illustrated  by  beds  of  clams  and  oysters,  the  schools  of  squid, 
etc. 

Notwithstanding  the  low  organization  and  sluggishness  of 
a  large  portion  of  the  branch  Mollusca,  we  are  compelled  to 
consider  that  it  has  been  a  very  successful  group  in  that  it 
has  held  its  place  with  practically  equal  numbers  through  the 
geological  ages,  and  has  succeeded  in  adapting  itself  to  the 
changes  of  those  ages.  Of  no  less  interest  is  the  additional 


252 


ZOOLOGY. 


fact  that  there  is  scarcely  a  nook  into  which  they  have  not 
penetrated,  except  where  continuous  drouth  prevails.  On  the 
other  hand,  it  is  among  the  more  active  types — the  cephal- 
opods — that  the  ancient  geological  forms  have  least  success- 
fully adapted  themselves  to  modern  conditions.  The  cephal- 
opods  appear  much  less  numerous  and  varied  now  than  in 
earlier  geological  time. 

295.  Classification. — The  following  are  the  principal  classes: 
Class  I.  Pelecypoda  or  Lamellibranchiata    (Mussels,   Oysters,   etc.). — 
Lamellibranchs    are    mollusks    in    which    the    fundamental    bilateral    sym- 

FIG.  114. 


.-^^-- 


FIG.  114.     Mya  arenaria,   a  burrowing  clam.     The  siphon  is  represented   as  fully   ex- 
tended.    This  is  quickly  retracted  when  the  animal  is  disturbed.      (After  Kingsley.) 

Questions  on  the  figure. — What  is  the  function  of  the  much  elongated 
siphons?  Which  is  the  anterior  end  of  the  animal?  Which  the  dorsal 
side?  What  would  seem  to  be  the  chief  function  of  the  foot  in  this  case? 


MOLLIJSCA.  253 

metry  is  shown  in  the  right  and  left  valves  of  the  shell  secreted  by  a 
bilobed  mantle,  and  in  some  of  the  internal  organs.  There  may  be  one 
or  two  adductor  muscles.  The  head  is  undeveloped.  The  ventral  body 
region  is  differentiated  into  a  muscular  foot,  shaped  like  a  plow-share. 
The  gills  are  in  sheets  (see  §285)  usually  two  on  either  side,  and  are  sus- 
pended in  the  mantle  cavity.  Paired  labial  palps  occur  about  the  otherwise 
unspecialized  mouth.  The  three  pairs  of  ganglia, — the  cerebro-pleural, 
the  pedal,  and  the  visceral, — are  usually  well  separated.  The  heart  con- 
sists of  two  auricles  and  one  ventricle  surrounded  by  a  pericardial  space, 
which  is  a  portion  of  the  body  cavity  and  communicates  with  the  exterior 
by  a  pair  of  nephridial  tubes.  The  reproductive  organs  are  simple;  the 
sexes  are  ordinarily  separate.  Development  by  a  metamorphosis. 

[The  primary  subdivisions  of  the  group  may  be  made  on  the  basis 
either  of  the  gill  structure,  the  adductor  muscles,  or  the  presence  or 
absence  of  the  siphon.] 

Order  i.  Isomya :    Two  adductor  muscles  which  are  essentially  equal. 

(a)  Siphon  well  developed,  retractile;  pallial  line  (Fig.  107)  with  a 
sinus.  Here  occurs  Mya  arenaria,  the  common  clam  of  the  Atlantic  coast. 
Great  heaps  of  shells  of  this  clam  show  that  it  was  much  used  by  the 

FIG.  115. 


FIG.    115.     Ensis  americanus,  the  razor  clam.     From  Verrill,  after  Gould. 

Questions  on  the  figure. — Where  is  the  hinge,  the  umbo,  etc.  ?  Trace 
the  lines  of  growth  and  compare  with  other  figures  of  bivalves. 

Indian  tribes  as  food.  In  New  England  the  clam  fisheries  are  of  very 
considerable  importance.  Mya  burrows  in  the  mud,  using  its  long  siphon 
to  keep  it  in  connection  with  the  water  from  which  it  gets  its  food.  Of 
somewhat  similar  habits  is  the  razor-shell  clam  (Fig.  115).  The  "borer" 
(Pholas)  and  the  "ship-worm"  (Teredo}  belong  to  this  group  and 
possess  the  power  of  boring  into  wood  or  stone  and  are  thus  of  much 
damage  to  submerged  structures  in  waters  where  they  abound. 

(fr)  Siphon  usually  present  but  not  highly  developed;  no  pallial  sinus. 
In  this  group  are  embraced  the  more  abundant  fresh  water  mussels  (Unio, 
Anodonta,  Cyclas},  and  the  cockles  (Cardium}  of  the  ocean.  The 
Unionidae  are  very  widely  distributed  and  very  common  in  our  own  fresh 
waters.  They  are  not  much  used  for  food  at  present,  though  the  Indians 
used  them,  probably  in  times  of  scarcity  of  other  food.  Their  shells  are 
widely  employed  in  the  making  of  buttons,  knife  handles  and  the  like, 


254 


ZOOLOGY. 


and  pearls  of  value  are  not  of  infrequent  occurrence.  These  are  merely 
the  mother-of-pearl,  which  ordinarily  lines  the  shell,  secreted  about  a  grain 
of  sand  or  other  irritating  object  which  finds  its  way  between  the  mantle 
and  the  shell.  Great  quantities  of  these  pearls  are  sometimes  found  in 
the  graves  of  the  mound  builders. 

FIG.  116. 


FIG.    116.     Mytilus    edulis,    a    Mussel.     From    Binney's    Gould. 

Questions  on  the  figure. — Identify  the  umbo.  What  are  your  evi- 
dences that  it  is  the  umbo?  Compare  the  lines  on  the  shell  with  those  in 
figure  117.  What  is  the  significance  of  the  specific  name  (edulis)?  What 
are  the  habits  of  the  species? 

Order  2.  Heteromya :  Two  adductor  muscles,  the  anterior  much  re- 
duced; siphon  usually  wanting.  Here  are  included  the  horse-mussel 
(Modiola)  and  Mytilus,  edible  mussels  which  occur  in  clusters  just  below 
low  tide  mark;  also  the  pearl-oyster,  from  which  the  best  pearls  are  taken. 
The  last  mentioned  form  is  not  found  on  our  own  coasts. 

Order  3.  Monomya :  One  adductor  muscle  (posterior)  ;  no  siphon. 
The  genus  Ostrea  (oyster)  and  the  genus  Pec  ten  (scallop)  are  the  most 
interesting  and  important  representatives  of  this  order.  The  species  of 
Ostrea  differ  much  in  size  in  different  regions.  The  largest  living  species 
is  a  Japanese  form  which  is  known  to  reach  a  length  of  two  to  three  feet. 
The  oyster  is  hermaphrodite.  The  young,  after  a  short  free  life,  become 
attached  by  one  of  the  /alves.  The  oyster  constitutes  a  larger  element  in 
the  food  supply  of  man  than  any  other  invertebrate.  The  scallops  are 
not  attached,  and  swim  by  a  rapid  opening  and  closing  of  their  valves. 

Class  II.  Gasteropoda  (Snails,  Slugs,  Whelks,  and  Periwinkles). — 
Gasteropods  are  mollusks  with  unsymmetrical,  univalved,  usually  spiral 
shells  (occasionally  lacking  the  shell  altogether).  The  head  and  foot 


MOLLUSCA. 


255 


ordinarily  preserve  the  bilateral  symmetry,  but  the  other  organs  lose  their 
symmetry  both  from  the  spiral  form  of  the  shell  and  from  a  twisting 
which  many  of  the  forms  undergo  by  which  the  nervous  system  and 
certain  other  visceral  organs  lose  their  original  right  and  left  relations. 
The  head  region  is  well  developed,  having  tentacles,  eyes,  and  a  mouth 
with  a  tooth-bearing  radula.  Gills  in  the  mantle  cavity  two,  one,  or  none ; 
in  the  air-breathing  forms  there  may  be  merely  a  pulmonary  sac.  The 
sexes  are  separate  (Streptoneura)  or  united  in  one  individual  (land  snails). 
Development  is  mostly  indirect. 

FIG.  117. 


FIG.   117.     Pecten  irradians, — a  Scallop.     From  Binney's  Gould. 

Questions  on  the  figure. — Is  this  an  external  or  internal  view  of  the 
shell?  Where  is  the  umbo?  What  is  peculiar  about  the  hinge  in  this  case? 
What  is  the  significance  of  the  lines  nearly  concentric  with  the  margin? 
Of  the  radial  lines? 

Subclass  I.  Streptoneura.—Gasteropods  in  which  the  nerve  loop  made 
by  the  visceral  commissures,  is  twisted  in  development  into  the  form  of 
the  figure  8;  the  other  visceral  organs  are  twisted  so  that  right  and  left 
are  interchanged.    Only  one  pair  of  tentacles  on  the  head.     Sexes  sc, 
Gills  usually  in  front  of  the  heart. 

One  of  the  common  representatives  ot  this  group  is  Littorina,  the 
common  periwinkle  of  the  seashore.  Many  other  types  of  almost  infinite 
variety  of  form,  size,  and  color  inhabit  the  ocean,  their  shells  often  being 
washed  ashore  by  the  waves;  such  are  the  cowries,  the  whelks,  the  cone- 
shells,  etc.  Here  belong  the  uncoiled  Limpet  and  the  slightly  coiled 
Crcpidula  or  boat-shell. 


ZOOLOGY. 


Subclass  II.  Euthyncura  (Land  Snails  and  many  naked  Mollusks). — 
Gasteropods  in  which  the  nerve  loop  is  not  twisted.  The  head  usually 
bears  two  pairs  of  tentacles.  The  sexes  are  united  in  the  same  individual. 
The  most  important  of  these  are  the  Pulmonata  or  air  breathing  Gastero- 
pods, some  of  which  are  terrestrial  and  others  aquatic.  Of  the  terrestrial 
snails  the  genus  Helix  (Fig.  119)  is  the  most  widely  distributed  and  inter- 
esting. Its  variability  is  such  that  between  three  and  four  thousand 

FIG.  1 1 8.  FIG.  119. 


FlC.   118.     Acmcea   testudinalis    (Limpet),    from   Binney's   Gould.     Upper   figure    lateral 
view;  lower  figure,  dorsal  view. 

Questions  on  the  figure. — How  do  the  Limpets  differ  from  the  ma- 
jority of  the  snails?  What  is  the  appropriateness  of  the  specific  name 
(testudinalis)  ? 

FIG.    119.     Helix  albolabris,   a  pulmonate   Gasteropod.     From   Binney's  Gould. 

Questions  on  the  figure. — What  is  the  significance  of  Helix?  Of 
albolabris?  Identify  the  parts  of  the  shell.  Is  it  a  right  or  left  spiral? 
What  do  you  mean  by  youi;  answer? 

FIG.  1 20. 


FIG. 


Limax  flavus,  a  Slug.     From  Binney's  Gould. 

Questions  on  the  figure.— How  do  the  ,ugs  differ  from  the  other 
Gasteropods  ?  In  what  external  respects  do  they  appear  similar  to  them  ? 
Compare  all  the  figures  of  slugs  you  may  be  able  to  find. 


MOLLUSCA.  257 

species  have  been  described.  Limax  (Fig.  120)  is  a  pulmonate  form  in 
which  the  shell  is  practically  wanting.  It  is  especially  destructive  to  cer- 
tain types  of  plants  as  it  is  a  voracious  vegetable  feeder.  The  aquatic 
pulmonates  are  represented  by  the  "pond-snail"  (Limnoea),  and  by 
Planorbis,  a  snail  whose  coils  are  in  one  plane,  presenting  a  helix  rather 
than  a  spiral. 

Class    III.  Cephalopoda     (Squid,    Devil-fish). — The    cephalopods    are 
bilaterally   symmetrical   mollusks    with    a    well-developed    head   in   which 

FIG.  121. 


FIG.  121.  Pearly  Nautilus.  From  Nicholson,  e,  eye;  h,  hood,  a  muscular  portion 
of  the  foot  which  protects  the  softer  parts;  s,  siphon;  se,  septa,  separating  the  succes- 
sive chambers  of  the  shell;  sp,  siphuncle;  t,  tentacles. 

Questions  on  the  figure. — How  does  this  shell  compare  with  those  of 
the  Gasteropods  f  What  is  considered  to  be  the  homology  of  the  tentacles 
or  arms  in  Cephalopods?  What  is  the  siphuncle?  What  is  the  character 
of  the  eye  in  Nautilus? 

the  front  part  of  the  foot  surrounds  the  well-armed  mouth  as  a  series 
of  lobes  or  tentacles.  The  head  protrudes  permanently  from  the  mantle 
cavity,  leaving  the  mantle  surrounding  the  posterior  part  of  the  body.  The 
posterior  lobe  of  the  foot  forms  a  siphon,  communicating  with  the 
mantle  cavity.  Into  this  cavity  the  nephridia,  the  aim-,,  and  the  reproduc- 
tive glands  open,  and  in  it  the  gills  lie.  The  shell  may  be  present  and  ex- 
ternal (Nautilus),  internal  and  slightly  developed  (Squid),  or  wanting 
(Octopus).  An  internal  cartilaginous  skeleton  protects  the  brain.  The 
coelom  is  well  developed.  The  ganglia  of  the  nervous  system  are  massed 
in  the  head  region.  The  sexes  are  separate  and  tr  development  direct. 
The  Cephalopoda  are  to  be  looked  upon  as  the  most  highly  developed  of 
the  Mollusca.  They  are  little  in  evidence  now,  however,  as  compared  with 
earlier  times. 

Subclass  I.  Tetrabranchiata. — Cephalopoda  in  which  the  front  segment 
of  the    foot   is    divided   into   lobes   bearing  numerous   tentacles,    without 
18 


258 


FIG.  122.  The  Devil-fish  (Octopus).  From  Cooke,  after  Merculiano.  A,  at  rest; 
B,  swimming,  a,  arms,  with  suckers  on  the  inner  aspect;  e,  eye;  s,  siphon  or 
funnel. 

Questions  on  the  figure. — Which  is  the  anterior^  end  of  the  animal  ? 
What  is  the  position  of  the  mouth?  What  is  the  function  of  the  siphon? 
Of  what  structure  is  it  a  part? 

FIG.  123. 


FIG.    123.     The  Paper  Nautilus  (Argonauta  argo).     From  Cooke,  after  Lacaze-Duthiers. 
e,  eye;   m,  mouth;   f,   siphon;   sh,   shell;   t,  tentacles. 

Questions  on  the  figure. — In  what  way  does  the  siphon  serve  in  loco- 
motion? In  which  direction  will  the  animal  move  by  means  of  the 
siphon?  How  does  the  shell  of  Argonauta  differ  from  that  of  Nautilus  f 


MOLLUSCA.  259 

suckers.  Shells  external  and  chambered  (and  in  Nautilus,  the  only  living 
genus,  coiled).  Two  pairs  of  auricles;  two  pairs  of  gills;  two  pairs  of 
nephridia. 

This  group  is  important  for  its  extinct  rather  than  for  its  living  repre- 
sentatives. The  pearly  or  chambered  nautilus  (Fig.  121)  found  in  the 
Pacific  and  Indian  Oceans,  is  the  only  important  living  species.  The 
Nautilus  appears  to  be  the  only  remaining  descendant  of  the  once  numer- 
ous family  of  Ammonites  and  more  remotely  still  of  the  Orthoceratites,  the 
rulers  of  the  Palaeozoic  seas  (see  Geology). 

Subclass  II.  Dibranchiata. — Cephalopods  in  which  a  circlet  of  8  to  10 
arms  surround  the  mouth.  These  bear  sucking  discs.  Shell  internal  and 
rudimentary  or  absent.  One  pair  of  gills,  one  pair  of  nephridia,  and  one 
pair  of  auricles.  An  ink  gland  is  present. 

Order  I,  Decapoda,  embraces  the  cuttle-fish  and  squid. 

Order  2,  Octopoda,  embraces  the  devil-fishes  (Fig.  122)  and  the  paper 
nautilus  (Fig.  123). 

296.  Supplementary  Studies  for  Library,  Laboratory, 
and  Field. 

1.  Compare  the  clam,  snail,  and  squid  with  regard  to  the 
following  particulars,  putting  the  results  in  a  tabular  form : 

(a)   Degree  of  development  of  the  head. 

(&)   Shell,  development  and  method  of  using,  in  each. 

(c)  Mantle;  extent,  form  and  modifications:  mantle  cavity. 

(d)  Foot;  parts,  differentiation,  and  uses. 

(e)  Respiration;   how   accomplished? 

(/)  Sense  organs;  position,  character,  and  degree  of  de- 
velopment. 

(g)  Locomotion;  how  effected? 
(h)   Protection;  special  devices. 

2.  Can  you  find  any  indication  among  the  mollusks  of  a 
relation  between   the   degree   of   development   of   the   sense 
organs  and  the  activity  shown  by  the  animals?    Between  the 
external  protective  structures  and  activity? 

3.  When  did  the  various  classes  of  mollusks  make  their 
appearance  in  the  history  of  the  earth?  (See  geology.)     What 
can  you  say  of  their  importance  in  the  formation  of  the  sedi- 
mentary rock? 

4.  In  what  ways  may  the  fresh-water  forms  have  arisen 
from  the  original  salt-water  mollusks  ? 


26O  ZOOLOGY. 

5.  What  members  of  the  group  of  mollusks  are  economically 
important?  Indicate  in  what  way  and  to  what  extent? 

6.  A  report  on  all  the  mollusks  to  be  found  in  your  com- 
munity; their  distribution,  habits,  etc. 

7.  Formation  of  pearls,  and  pearl  fisheries. 

8.  The  industries  connected  with  the  use  of  the  shells  of 
the  clam. 

9.  The  life  history  of  the  fresh-water  clam. 

10.  The  life  history  of  the  oyster. 


CHAPTER    XVII. 
PHYLUM  VII.— ARTHROPODA. 

297.  This  group  is  one  especially  favorable  for  the  pupils 
to  study  in  the  field,  in  the  haunts  of  the  animals  themselves. 
For  this  reason,  wherever  it  is  at  all  possible,  the  members  of 
the  class  should  be  required  to  collect  a  portion  of  the  material 
needed  in  the  laboratory  and  to  submit  a  report  on  such  items 
of  physiology  and  ecology  as  may  be  expedient  in  each  case. 
The    teacher    will    find    suggestions    in    the    supplementary 
exercises. 

298.  The  Fresh-water  Cray-fish  (Cambarus). — This  form 
should  be  studied  when  living  specimens  may  be  had.     They 
may  be  kept  for  considerable  time  in  a  tub  containing  an  inch 
of  water.     This  should  be  changed  every  day  or  two.     Feed 
on  small  pieces  of  meat  or  earthworms. 

I.  Physiology. 

1.  Locomotion:  walking;  how  effected?    Swimming;  how 
effected?    Under  what  circumstances  does  the  animal  swim? 
Do  all  the  walking  legs  act  together  in  walking?   How  many 
are  at  rest  at  once?   In  what  order  do  they  act? 

2.  Movements  of  the  parts  of  the  body:  segments,  and 
appendages.     Describe  the  manner  and  purpose  of  these  mo- 
tions as  far  as  you  can  determine.    In  what  different  ways  do 
the  various  groups  of  appendages  seem  to  act?   Watch  them, 
one  pair  at  a  time. 

3.  Feeding :  kind  of  food  used  and  manner  of  securing  it. 

4.  Respiration:  by  means  of  air  or  water?    How  can  you 
be  sure?    Does  the  animal  do  anything  to  renew  the  water, 
by  producing  currents?    Place  a  minute  amount  of  carmine 
or  indigo  solution  at  the  side  of  the  animal  at  the  union  of 
the  abdomen  and  thorax;  at  the  front  of  the  thorax.    What  is 
the  difference?   What  does  it  signify? 

261 


262  ZOOLOGY. 

5.  Evidences  of  sensitiveness :  Devise  experiments  of  your 
own  to  prove  whether  the  cray-fish  is  stimulated  by  light; 
contacts;  the  presence  of  food  in  any  other  way  than  by  sight; 
sound.  Are  all  parts  of  the  body  equally  sensitive  to  touch? 
To  chemical  stimuli?  Make  use  of  a  5  per  cent,  solution  of 
acetic  acid;  strong  salt  solution;  strong  beef  extract.  What 
inferences  may  be  drawn  from  your  experiments? 

II.  Symmetry. —  (This   group   is   especially    favorable    for 
this  study.) 

Notice  what  is  implied  in  bilateral  or  tri-axial  symmetry. 

Antero-posterior  axis ;  are  the  poles  alike  or  different  ? 

Make  a  memorandum  of  all  the  chief  differences. 

Dorso-ventral  axis  (as  above). 

Right-left  axis.     Record  the  points  of  agreement. 
Contrast  the  axes  in  length.     Can  you  think  of  any  causes 

for  the  differences  and  likenesses  discovered  above  ?  Any 

advantages  arising  therefrom  ? 

III.  General  Form. — Distinguish   two   regions; — Cephalo- 
thorax  and  abdomen. 

Cephalo-thorax ;  carapace. 
Head;  rostrum,  eyes,  mouth. 
Cervical  groove. 
Thorax. 

Abdomen;  how  many  segments  do  you  find?  What  seems 
to  determine  a  segment? 

Applying  these  criteria  can  you  find  any  indications  of  seg- 
mentation in  the  cephalo-thorax ?  (Make  a  temporary 
estimate  of  the  number  of  segments  in  the  animal.) 

Make  two  sketches  showing  a  dorsal  and  a  ventral  view  of 
the  cray-fish,  preserving  proportions. 

Examine  one  of  the  abdominal  segments  (the  third  or 
fourth  from  the  front).  How  is  it  joined  to  those  next 
it?  Follow  the  line  of  union.  Note,  tergum,  or  dorsal 
piece;  sternum,  or  ventral  piece;  pleura,  the  lateral  pro- 
jections from  the  tergum. 


ARTHROPODA. 


263 


Make  a  sketch  of  an  imaginary  cross-section  showing  the 
relation  of  these  parts  to  each  other,  together  with  the 
attachment  of  the  appendages. 

IV.  Appendages. — Group  them  into  regions  and  notice  the 
general  differences  and  the  differences  in  the  uses  to 
which  they  are  put.  If  time  will  allow,  study  the  ap- 
pendages in  detail  as  follows  : 

1.  Begin  with  the  third  or  fourth  abdominal  appendage   (swimmerets) 
making  the  drawings  necessary  to  show  the  parts : 

Protopodite,  or  basal  portion. 

Exopodite,  or  external  branch. 

Endopodite  or  internal    (median)   branch. 

Compare  all  the  abdominal  segments  with  that  studied.  Do  different 
individuals  agree  in  the  appearance  of  the  first  and  second  abdominal 
segments?  Compare  the  last  segment  (telson)  with  those  studied.  How 
many  segments  in  the  abdomen?  Of  what  parts  is  the  tail  fin  made  up? 

2.  Cephalo -thoracic    Appendages. — Remove     with     scissors     the    over- 
arching portion  of  the  carapace  and  expose  the  base  of  the  appendages. 
Find  the  third  maxilliped  (the  first  appendage  in  front  of  the  large  claw). 
Remove  by  inserting  a  scalpel  and  bringing  away  all  that  belongs  to  it. 

Identify : 

Protopodite,    of    two    segments    (coxopodite,    next    the    body,    and 

basipodite). 

Endopodite  and  exopodite.     How  many  pieces  in  each? 
Epipodite,    lying   in   the   gill-chamber.      Are   there   any   special   out- 
growths  on  it  ? 

Study  and  compare  with  this  the  large  claw,  and  the  other  walking 
appendages.  Which  part  is  wanting  in  these,  exopodite  or  endo- 
podite?  Reasons  for  your  view?  How  do  these  five  appendages 
differ  frpm  each  other. 

Examine  and  compare  the  appendages  in  front  of  the  third  maxilliped 
in  order : 

Second  maxilliped. 

First  maxilliped. 

Second  maxilla.      1 

First  maxilla. 

Mandible.  fHead  Parts" 

Antenna. 

Antennule. 

What  are  the  evidences  that  the  antennae  and  antennules  are  homolo- 
gous with  those  already  described? 
Revise  your  estimate  of  the  number  of  segments. 


264  ZOOLOGY. 

Compare  the  appendages  again  by  groups,  and  notice  the  chief  points 
of  difference,  and  the  ends  served  by  these  differences.  Make  a 
careful  sketch  of  each  type  of  appendage,  labeling  all  parts.  (The 
names  of  the  segments  of  the  larger  appendages  may  be  found  in 
fuller  texts,  if  desired.) 

By  studying  the  living  specimen,  determine  just  the  work  done  by 
each  of  the  types  of  appendages. 

Note  the  position  of  the  eyes.     Examine  with  a  low  power. 

In  the  basal  joint  of  each  antenna  is  the  opening  of  the  "green  gland." 

In  the  basal  joint  of  the  antennules  are  the  otocysts. 

V.  Gills. — Examine  the  gill-chamber,  and  the  position  of 
the  gills  therein.     Which  appendages  bear  gills  ?    How  many 
tufts  to  each  appendage?   How  do  they  differ  as  to  the  place 
of  their  attachment?  How  many  in  all?  Make  a  table  showing 
these  facts. 

VI.  Internal  Organs. — Remove  with  much  care  the  carapace  from  the 
thorax  and  the  terga  from  the  abdominal  segments,  by  the  use  of  scissors 
and  forceps.    Sketch  the  organs  in  their  natural  position.    What  organs  are 
visible? 

Examine  in  some  detail  the  following  sets  of  organs, 
(a)  The  circulatory  organs. 
Heart:  just  beneath  the  carapace,  in  a  membranous  chamber  (peri- 

cardial  sinus). 
Apertures,   by   which   the   blood   enters   the   heart   from   the   sinus ; 

dorsal,  ventral,  lateral.     How  many  do  you  find? 
Arteries ;  anterior,  posterior. 
(The  teacher  should  have,  if  possible,  a  permanent  preparation  of  the 

lobster  in  which  the  arterial  circulation  has  been  injected  with  a 

colored  mass.) 

VII.  Reproductive  Organs. — These  will  be  found  immediately  beneath 
the  pericardial  sac  as  whitish    (male),  or  yellowish  to  brown    (female) 
lobed  structures.    Depending  on  the  sex  there  will  be  found 

Ovaries  or  testes.     Form,  position,  and  number  of  lobes? 
Oviducts  or  vasa  deferentia.     Course,  length  and  outlets? 
Can  you  determine  the  sex  of  your  specimen?    Note  especially  the  ex- 
ternal differences  between  males  and  females. 

VIII.  Digestive  Organs. 

Liver,  a  pair  of  yellow,  brown  or  reddish  masses  anterior  to  the  re- 
productive organs. 

Stomach;  sketch  in  position.     Dissect  later,  if  time  allows,  and  note 
the  anterior  and  posterior  chambers,  and  the  grinding  apparatus. 

How  is  the  mouth  situated  relatively  to  the  stomach? 

Follow  the  intestine  backward  from  the  stomach  to  the 
Anus:  position  of? 


ARTHROPODA.  265 

Make  a  sketch  of  the  entire  tract  from  a  side  view,  showing  in  what 
part  of  the  carapace  each  portion  is. 

IX.  Muscular  System. — How  is  the  abdomen  flexed  and  how  extended? 
How  do  the  muscle  fibres  run?     To  what  attached?    Are  they  plain  or 
striate?   How  are  the  appendages  worked?    Split  open  the  segments  of  the 
chela. 

X.  Nervous  System. — (If  the  time   is  short  a  demonstration  may  be 
made  by  the  teacher,  preferably  with  a  lobster.) 

Remove  the  intestine,  and  cut  carefully  through  the  muscles  in  the 
median  line  until  the  white  ventral  nerve-chain  is  uncovered.  Follow  it 
forward  to  the  head,  cutting  away  the  covering  plates  in  the  thorax. 

How  many  swellings  (ganglia)  in  the  abdominal  region?  Relation  to 
the  segments?  Where  do  nerves  arise? 

Thoracic  ganglia :  number  and  relation  to  appendages  ? 

Suboesophageal  ganglion;  circumcesophageal  connective. 

Supracesophageal   ganglion    (brain). 

Do  any  nerves  arise  from  the  brain?  Where  distributed?  Draw  from 
above. 

Make  a  diagram  of  the  relation  of  the  digestive  tract  and  nervous  sys- 
tem from  the  side  view. 

XI.  Excretory  Organs. — The  green  glands  occur  at  the  base  of  the  head, 
in  front  of  the  mouth.    The  outlets  are  at  the  base  of  the  antennse. 

Make  a  diagrammatic  view  of  an  imaginary  cross-section  of  the  thorax 
in  the  region  of  the  heart,  and  one  of  the  abdomen,  showing  the  position 
of  the  internal  organs.  Also  a  diagram  of  a  sagittal  section  showing  rela- 
tions of  all  the  parts  discovered. 

299.  Sowbug  (Oniscus,  a  terrestrial  form;  or  Asellus,  a  fresh  water 
Isopod). — 

General  Form. — Use  hand  lens  and  identify : 
Head:  size,  form,  number  of  segments. 

Eyes :  number  and  position. 

Antennules  and  antennse. 

Mouth-parts :  number  and  structure. 

Thorax:  number  of  segments.     What  variation  therein? 
Abdomen.     How  many  segments?    Proofs? 
Appendages. 

Remove  carefully,  mount  in  water  on  a  slide,  and  examine  with  low 

power,  a  thoracic  appendage.     Sketch. 
Examine  similarly  the  other  thoracic  legs  and  the  mouth  parts,  and 

make  drawings  of  them  arranged  in  the  order  of  their  occurrence. 
Examine  similarly  the  abdominal  appendages.    What  is  their  number? 

Sketch. 
Compare  the  appendages  from  the  different  regions,  as  to  structure, 

form  and  probable  function.    Are  there  any  gills?  Where  situated? 
What  is  the  number  of  segments  in  the  body,  if  there  is  a  pair  of 

appendages  to  each  segment? 


266  ZOOLOGY. 

Comparisons. — 'Compare  the  sow-bug  with  the  cray-fish  as  to  the 
degree  of  union  of  head  and  thorax;  the  number  of  segments  represented 
in  each  of  the  three  regions;  the  degree  of  differentiation  among  the 
appendages;  the  mode  of  respiration;  the  presence  of  both  exopodite  and 
endopodite;  as  to  food,  and  habits. 

Physiology  and  Ecology. — A  study  and  report  of  the  animal's  habitat, 
food  habits,  methods  of  motion,  sensitiveness  to  light  and  to  other  classes 
of  stimuli,  should  be  made.  How  does  Oniscus  behave  when  touched? 
Do  you  find  any  trace  of  eggs  or  young?  What  facts  are  to  be  noted  con- 
cerning them? 

300.  Cyclops. — These   minute   freshwater   Crustacea   may  be  found   in 
almost  any  pool  where  aquatic  plants  are  found.     They  flourish  well  in 
aquaria.     Select  several  of  the  larger  specimens  with  egg  masses  one  on 
each  side  the  abdomen.     Examine  in  a  watch  glass  with  a  little  water  to 
which  a  drop  of  chloroform  has  been  added.    Use  low  power  of  microscope. 

General  Form. — (Study  both  dorsal  and  ventral  surface.) 
Cephalo-thorax : 

Anterior  portion  covered  with  the  carapace.     How  many  segments 
represented  ?   How  can  we  know  that  this  is  not  merely  the  head, 
or  the  whole  cephalo-thorax  ? 
Posterior  portion  (four  free  thoracic  segments).    How  is  it  known 

that  these  are  not  abdominal  segments? 
Abdomen :  form ;  number  and  character  of  the  segments. 
Appendages. — Antennae,  oral,  thoracic,  abdominal.    Number  and  general 
character  of  each.    Where  and  how  are  the  egg-cases  attached? 
Sense  Organs. 

Eye  spot  (appearing  as  one,  from  which  the  name  Cyclops). 
Do  you  find  any  organs  which  suggest  a  tactile  function? 
Report  on  all  available  points  of  physiology:  as  food  habits;  methods 
of  locomotion;   reaction  to  light  and  other  stimuli. 

301.  Comparisons. — Collect  all  the  minute  fresh-water  Crustacea  pos- 
sible and  compare  them  with  Cyclops.     Learn  to  identify  them  by  their 
manner  of  moving  in  the  vessels  of  water.    Daphnia  is  especially  favorable 
for  microscopic  study  on  account  of  its  semi-transparency. 

302.  Spider  (any  common  species  large  enough  for  study). 
General  Form. — Study  the  relations  of  head,  thorax,  and 

abdomen.    Are  there  any  antennae?  Oral  appendages?  Num- 
ber and  character  of  the  thoracic  appendages?   Does  the  ab- 
domen show  any  signs  of  segmentation?   Has  it  any  append- 
ages ?  Make  sketches  showing  a  ventral  and  a  lateral  view. 
Special  Organs. 

Examine  the  head  with  a  hand  lens  and  locate  the  eyes. 

Note  more  particularly  the  types  of  appendage  found, 


ARTHROPODA.  267 

and  the  degree  of  differentiation.     Find  the  openings 
to  the  air-sacs  on  the  ventral  surface  of  the  abdomen. 
Locate  the  spinning  glands.     Number? 
Activities  and  Habits. — How  do  the  legs  act  in  walking? 
At  what  joints  are  they  flexed  at  various  parts  of  the  step? 
Do  all  the  legs  on  one  side  act  in  unison?    Observe  the  spin- 
ning action.     Does  the  spider  ever  produce  the  threads  except 
when  weaving  a  web?    Describe.     Determine  if  possible  the 
kind  of  web  formed  by  the  species  studied.     Or  find  as  many 
types  of  nest  or  web  as  possible  and  learn  to  recognize  the 
spiders  producing  them.     How  does  the  spider  travel  on  its 
web  ?  Where  do  spiders  place  their  webs  ?   Place  a  fly  or  other 
insect  in  a  newly  constructed  web  and  record  the  actions  of 
the  spider.    Can  you  devise  means  to  prove  whether  the  spider 
possesses  the  sense  of  smell? 

303.  The  Grasshopper. — Several  species  of  the  locusts 
may  be  found  in  almost  every  locality.  They  are  especially 
abundant  in  the  early  autumn.  For  laboratory  study  select 
the  largest  species  found  in  sufficient  abundance.  In  connec- 
tion with  the  securing  of  material  the  students  should  make 
observations  on  the  following  points: 

1.  Habits. — Where  and  under  what  circumstances  found? 
At  what  time  of  the  year  does  this  species  occur  in  greatest 
abundance  ?    Under  what  circumstances  are  they  most  active  ? 

2.  Methods   of  Locomotion. — How   many   methods   seem 
available?  Degree  of  efficiency  of  each?  Under  what  circum- 
stances is  each  used?    What  distance  can  be  attained  at  one 
effort?   Continue  the  study  later  in  more  limited  quarters,  as 
in  the  room  and  under  a  bell-glass.     Compare  the  work  of 
the  various  legs.     Are  the  wings  used  at  all  in  jumping? 

3.  Protective  Features. — Coloring;  to  what  extent  do  you 
find  this  of  protective  value?  Reasons.    Does  the  animal  show 
a  distinct  instinct  for  hiding?    Compare  all  available  species 
in  these  regards. 

4.  Do  they  produce  definite  sounds?    Under  what  circum- 


268 


ZOOLOGY. 


stances?   Do  you  find  any  hint  as  to  the  method  of  their  pro- 
duction ? 

5.  Do  you  detect  any  movements  which  suggest  respiration  ? 
Rate?    (Find  spiracles  in  the  thorax  and  abdomen.) 

6.  Supply  hungry  animals  with  fresh  leaves  nnd  study  the 
feeding  process.      Dip   the   leaves   in   various   solutions   and 
notice  whether  it  makes  any  difference  to  the  grasshopper. 

If  alcoholic  material  is  used  for  the  following  morphological 
studies  it  should  not  be  allowed  to  become  dry.  If  dipped  in 
a  mixture  of  glycerine  and  50  per  cent,  alcohol,  specimens 
will  not  dry  so  rapidly. 

The  sexes  differ,  particularly  in  the  abdominal  region. 
Procure  specimens  thus  differing  by  examining  a  number  of 
individuals,  and  keep  both  kinds  for  comparison.  Sketch 
dorsal,  lateral  and  ventral  views  of  each  (especially  in  the 
regions  of  difference). 

External  Features. — Study  the  following  points : 

1.  The  regions  of  the  body. 
Head;  thorax;  abdomen. 

What  are  the  signs  of  segmentation  in  these  three 
regions?  Where  is  it  most  clearly  indicated?  Where 
are  the  segments  most  similar? 

2.  Abdomen. 

Number  of  segments  (differs  in  male  and  female). 
Dorsal  and  ventral  plates.    Are  they  equally  developed  in 

all  segments. 
Appendages :  which  segments  possess  them  ? 

Ovipositors    (paired   outgrowths    found   only   in   the 

female). 

Anal  cerci   (examine  the  male).     Are  they  found  in 
the  female?  To  what  segments  do  these  appendages 
belong  ? 
Spiracles  (small  openings  at  the  side  of  the  segments)  ; 

number  and  distribution? 

Tympanic  membrane,  at  the  sides  of  the  first  abdominal 
segment. 


ARTHROPODA.  269 

3.  Thorax;  studying  from  the  front,  backward,  find: 
Prothorax;   mesothorax;  metathorax.      Note  the   form, 

size,  and  structure  of  each  part. 
Appendages  of  each  segment. 

Legs:  number;  relative  size;  parts  (beginning  at  the 
body),  coxa,  trochanter,  femur,  tibia,  tarsus.  Com- 
pare the  legs. 

Wings    (can  these  be  regarded  as  homologous  with 
the  jointed  appendages?)  :  number;  position,  at  rest 
and   in  motion;   characteristics;   position   of   veins. 
Compare  the  two  pairs  in  all  essential  particulars. 
Are  there  any  spiracles  in  the  thorax?    Position? 

4.  Head    (is  there  any  "neck"?).     The  head  is  covered 
with  chitinous  plates;  identify: 

Epicranium,  the  dorsal  plate. 

Clypeus,  the  anterior  plate. 

Gense,  the  lateral  plates. 

Labrum  or  upper  lip,  anterior  to  the  clypeus. 
Examine  the  compound  eyes,  their  form  and  relation  to  the 

plates.     Slice  off  a  portion  of  the  surface  and  study  the 

surface  with  a  low-power  objective. 
Ocelli  or  simple  eyes.    How  many  and  in  what  position? 
Mouth  aperture;  position. 
Appendages  of  the  head: 
Antennae,  near  the  eyes;  number. 

Mouth-parts.  These  are  complicated  and  demand  careful  study,  if 
satisfactorily  made  out.  Remove  the  labrum  and  proceed  from 
before,  backward. 

Mandibles;  a  pair  of  horny  tooth-bearing  jaws.     Draw  in  position. 

Maxilla;   a  pair  of  compound  jointed  organs  made  up  of  three 

portions,  the  lacinia   (nearest  the  median  line),  the  galea,  and 

the  maxillary  palpus   (external). 

Labium  or  lower  lip;  this  also  bears  a  palpus.     The  labium  may 

be  studied  and  removed  before  the  study  of  the  maxillae. 
Tongue. 

How  many  segments  seem  to  be  represented  in  the  head? 
Internal  structure. 

'  Select  large  female  specimens  preferably.     Clip  the  wings  close  to 
the  body,  and  pin  the  specimen  to  a  board,  dorsal  surface  upward. 


270  ZOOLOGY. 

With  a  pair  of  fine,  sharp  pointed  scissors  make  a  longitudinal 
incision  into  the  integument  of  the  abdomen  near  each  side. 
Gradually  and  carefully  remove  the  skin  between  the  cuts  from 
behind  forward.  Look  for  the  heart, — a  long,  thin-walled,  mid- 
dorsal  vessel,  which  if  not  removed  with  the  skin  may  be  seen 
just  beneath  it.  Unroof  both  the  abdomen  and  thorax.  Note  the 
exposed  muscles  of  the  thorax,  also  the  whitish  fat  bodies  next 
the  body  wall. 

1.  Trachea. — If  the   specimen  is   freshly  killed  the  tracheae  will  be 
rilled  with  air  and  will  show  as  white,  glistening  tubes.    Seek  their 
connection    with    the    spiracles,    and    note    their    ramification    and 
unions  in  the  body.    Isolate  some  of  the  smaller  branches  and  study 
under  the  microscope.    Prove  that  they  are  tubes.    How  kept  open? 

2.  Reproductive    Organs. — (These    are    much    more    difficult    in    the 

male.) 

Ovaries:  In  how  many  masses?  Notice  the  subdivisions  of  the 
ovaries.  These  contain  the  eggs  and  communicate  by  means  of 
an  oviduct  with  the  outside.  In  what  segment?  Examine  an 
ovum  with  the  microscope.  Mash,  and  notice  the  yolk. 

3.  How  do  the  muscles  of  the  thoracic  region  differ  from  those  in 
the  abdominal?    Are  the  fibres  plain  or  striate? 

4.-  Digestive  Tube. 

Dissect  forward  into  the  head,  and  press  the  other  organs  aside  so 
that  the  course  of  the  tract  may  be  revealed.     It  consists  of 
the  following  parts,  which  should  be  identified. 
Mouth. 

(Esophagus;  size  and  course. 

Crop   (an  enlargement  of  the  oesophagus)  ;  shape,  position. 
Stomach :  character  and  extent.     (At  the  anterior  end  is  a  ring 
of  tubular  appendages  which  are  glandular  in  function, — the 
gastric  caeca;  at  the  posterior  end  it  is  limited  by  a  circle  of 
fine  tubes — Malpighian  tubules — which  are  excretory.) 
Intestine;  length,  course  and  size. 
Anal  opening;  position. 

Make  drawing  of  digestive  tract  from  side  view,  showing  in  outline 
the  body  regions  and  the  relation  of  the  portions  of  the  tract  to 
these. 
5.  Nervous  System. —  (Remove  the  alimentary  tube  and  examine  the 

floor  of  the  abdominal  cavity.) 
Ventral  nerve  cord.     Is  it  single  or  double? 

Ganglia;   number,  and  relation  to  the   segments. 
Nerves ;  origin  and  distribution. 
Trace  forward  into  the  thorax  and  head. 

Ganglia;  number  and  position.     How  connected?    Is  there  any 

portion  dorsal  to  the  digestive  tract  (brain)  ? 
Nerves. 


ARTHROPODA.  27 1 

Compare  the  nervous  system  of  the  grasshopper  part  by  part  with 

that  of  the  cray-fish. 

Make  diagrammatic  representations  of  imaginary  cross-sections 
through  thorax  and  abdomen  showing  the  relation  of  the  different 
structures :  likewise  of  a  sagittal  section. 

The  cricket   or  cockroach   may  be   substituted    for   or   compared   with 
the  grasshopper. 

304.  Supplementary  Laboratory  and  Field  Work. — It 

is  perhaps  inexpedient  for  students  in  an  elementary  course 
to  make  dissections  of  other  representatives  of  the  Arthropoda, 
but  the  common  air-breathing-  forms  are  so  numerous,  so 
varied,  and  have  such  interesting  habits  and  histories,  that 
they  may  profitably  be  used  as  a  basis  for  individual  field  and 
laboratory  work  and  to  serve  in  the  comparison  of  homologous 
organs  in  related  groups.  The  following  outlines  are  sug- 
gestive rather  than  exhaustive. 

I.  Make  a  table  in  which  can  be  displayed  the  points  of  con- 
trast between  the  cray-fish,  the  grasshopper,  the  "  June  bug  " 
(or  other  beetle),  the  squash-bug  or  the  cicada  ("locust  "), 
the  butterfly,  the  wasp,  the  fly,  the  spider,  and  the  centipede, 
in  the  following  particulars  : 

1.  The  regions  of  the  body;  head,  thorax,  and  abdomen; 
their  degree  of  development  and  separateness. 

2.  The  number  of  segments  in  the  body,  and  the  clearness 
with  which  they  are  manifest. 

3.  The  degree  of  diversity  shown  by  the  segments  in  the 
various  parts  of  the  body. 

4.  The   points   of   structure   which    the   various   segments 
possess  in  common. 

II.  Make  a   similar  table,   for  the   same   animals,   of  the 
appendages. 

1.  Head  appendages:  antennae;  mouth  parts,  number  and 
kinds. 

2.  Thoracic  appendages : 

Legs :  number,  position,"  kinds,  joints,  special  adaptations 
to  special  work. 


272  ZOOLOGY. 

\Yings :  number,  size,  position,  structure,  principal  veins. 
Compare  the  first  and  second  pairs  as  to  size,  struc- 
ture and  function. 
3.  Abdominal  appendages;  number,  structure,  function. 

III.  Make   a   table   comparing   these   and   other    available 
forms  as  to  their  eyes,  simple  and  compound. 

IV.  Find,  if  possible,  another  form  embodying  the  same 
general  features  found  in  each  of  the  above  mentioned  ani- 
mals. 

V.  Compare  these  (or  other  forms  which  may  be  selected) 
from  the  point  of  view  of  their  habits.     Introduce  all  discov- 
ered correlation  between  structure  and  function. 

1.  Haunts  and  place  of  living.     If  peculiarly  local,  can  you 
find  any  reasons? 

2.  Locomotion ;  methods,  and  the  efficiency  of. 

3.  Feeding;  material  used,  and  the  method  of  obtaining  it. 

4.  Respiration;    organs    and    their    location;    any    special 
points  as  to  their  use. 

5.  Special   senses;   physiological   evidences;   morphological 
evidences. 

6.  The  laying  of  eggs  and  provision  for  the  young. 
(The  library  may  be  used  profitably  to  supplement  field 

work  in  this  section.) 

VI.  Study  by  observation  the  homes,  temporary  or  perma- 
nent, their  mode  of  construction  and  uses,  in  the  following: 
Spiders   (as  many  species  as  possible),  the  paper-wasp,  the 
mud-wasp,  the  honey-bee,  the  bumble-bee,  ants,  flies,  etc. 

VII.  Development  or  life  history.     Studies  may  be  made 
in  natural  conditions  in  many  cases  by  periodic  observations. 
When  this  is  not  possible,  animals  may  often  be  reared  in  con- 
finement by  supplying  the  appropriate  conditions.     This  is  a 
very  attractive  line  of  investigation  and  one  in  which  real 
contributions  to  knowledge  may  be  made.     The  following  are 
some  of  the  matters  to  be  kept  in  mind. 

i.  Is  there  a  metamorphosis  or  is  development  direct ?    (See 
text  §  323.) 


UNIVERSITY 


ARTHROPODA.  273 

2.  Eggs  :  where  deposited  ?   In  what  numbers  ?   Relation  to 
future  food  supply? 

3.  Larval  condition   ("grub,"  "maggot,"  "caterpillar"). 
Form,  segmentation,  general  external  characters,  special 
organs  ;  habits,  food,  coloration,  enemies. 

4.  Pupa    (a   resting  and   transforming  stage)  ;   how   pro- 
tected ?   What  is  the  origin  and  character  of  the  protect- 
ing  structure?     What   changes    are   undergone   at   this 
stage? 

5.  Adult.     How  do  the  larva  and  pupa  compare  with  it  in 
segments,  appendages,  etc. 

The  following  forms  may  be  studied  and  compared  as  to  life 
history  : 

Squash-bug;  all  stages  are  to  be  found  on  squash,  gourd, 

cucumber  and  similar  vines. 
Potato  beetle;  equally  abundant  on  the  Irish  potato  plant 

in  some  years. 

Bees  and  wasps;  to  be  found  in  their  nests. 
"  Blue-bottle  "  fly.    This  form  may  be  studied  in  confine- 

ment.     (Expose  raw  meat  for  the  eggs  to  be  laid. 

Place  on  a  chip  in  a  dish  of  moist  earth  or'sand.    Invert 

a  tumbler  or  bell-  jar  over  it  and  watch  the  growth  and 

changes,  as  decomposition  proceeds.) 
Mosquito.     The  larvae  may  be  found  in  stagnant  pools, 

and  watched  in  confinement. 
Cabbage  butterfly.     This   form  may  be  studied  in  the 

garden,  or  in  the  laboratory  by  placing  tne  cctub^c 

leaves  with  the  larvae  under  a  bell-  jar  and  keeping  the 

conditions  favorable. 
Some  large  caterpillar  should  be  studied  with  some  de- 

gree of  care  in  order  to  ascertain  the  general  arrange- 

ment of  organs. 
Spider.    If  a  mass  of  spiders'  eggs  can  be  found,  the  stu- 

dent by  watching  may  be  able  to  determine  whether  the 

development  is  direct  or  indirect. 
19 


274  ZOOLOGY. 

Silk-worm.     The  various  stages  may  be  studied  in  con- 

finement. 

VIII.  Group  the  Arthropoda  known  to  you,  in  three  classes  : 
(i)  those  hurtful  to  man's  interests,  (2)  those  beneficial 
thereto,  and  (3)  the  harmless.  State  the  grounds  of  your 
classification  of  each  form.  In  what  stage  of  its  metamor- 
phosis is  each  species  hurtful  or  helpful.  Extend  your  own 
knowledge  by  inquiry,  by  observation,  and  by  reading. 

DESCRIPTIVE  TEXT. 

305.  The  group  of  Arthropoda  embraces  more  than  one- 
half  the  species  in  the  animal  kingdom,  and  is  correspondingly 
rich  in  individuals.  The  segmented,  bilaterally  symmetrical 
body  and  the  arrangement  of  the  nervous  system  are  the  most 
important  points  of  similarity  with  the  Annulata.  The  gen- 
eral resemblance  is  more  striking  in  some  of  the  lower  forms 
(Peripatus),  and  in  the  larval  stages  of  those  which  undergo 
a  metamorphosis.  The  subdivisions  of  the  phylum  (if  it  can 
be  considered  a  single  phylum)  are  quite  diverse  and  their 
relationships  uncertain.  There  are  many  parasitic  and  other- 
wise degenerate  forms  which  make  the  problem  of  classifica- 
tion more  difficult. 


General  Characters. 

1.  Elongated,  bilaterally  symmetrical  body. 

2.  Segmented  ;  somites  heteronomous,  and  typically  grouped 
into  three  regions;  (i)  head,  (2)  thorax,  (3)  abdomen. 

-  3.  An  outer  skeleton,  of  a  secreted  chitinous  substance. 

4.  Each  somite  has  typically  a  pair  of  jointed  appendages 
(whence  the  name  arthropod). 

5.  Central  nervous  system  similar  to  that  of  Annulata:  (i) 
brain,  (2)  a  nerve  ring  around  the  oesophagus  connecting  the 
brain  with  (3)  a  ventral,  ladder-like  chain  of  ganglia. 

6.  Heart,  dorsal  to  the  digestive  trace. 

7.  Coelom  represented   largely  by   secondary  blood   spaces 
connecting  with  the  circulatory  system. 


ARTHROPODA.  275 

307.  General  Survey. — The  s}tnmetry  of  the  Arthropods 
is  very  pronounced,  except  in  the  case  of  fixed,  parasitic,  or 
otherwise  J|generate  forms    (as  barnacles,  Sacculina,  etc.). 
The  group  presents  great  diversities  expressive  of  a  high  de- 
gree of  adaptation  to  almost  every  conceivable  mode  of  life. 
They  may  be  parasitic — internal  or  external,  symbiotic,  social, 
or  independent ;  they  may  be  aquatic,  terrestrial,  burrowing  or 
aerial;  they  use  all  sorts  of   food;  they  bore,  crawl,  swim, 
jump,  fly,  or  may  be  fixed.     In  geographical  distribution  they 
are  practically  cosmopolitan.     The  group  is  one  of  the  most 
successful  in  the  animal  series.     None  of  the  living  species, 
however,  attains  a  very  great  size.     The  king-crab  and  the 
lobster  are  among  the  largest.     Many  are  microscopic. 

308.  The  Segments. — There  is  a  great  deal  of  diversity 
among  the  segments  of  the  body  as  to  size,  shape,  the  form 
and  use  of  their  appendages,  as  well  as  in  their  contained 
structures.     In  the  more  primitive  forms  (Peripatus  and  the 
centipedes)  and  in  the  larval  condition,  the  somites  are  well 
marked  externally,  but  in  the  majority  of  forms  there  is  more 
or  less  fusion  of  contiguous  somites  in  certain  body  regions. 
A  variable  number  of  segments  at  the  anterior  end,  which 
bear  the  mouth  parts  and  sense  organs,  form  the  head.     Be- 
hind these  a  group  of  three  (insects),  or  more  (cray-fish),  may 
fuse   to    form  the  thorax.      These  two   regions,   head   and 
thorax,  are  often  fused  into  one  piece — the  cephalothorax. 
The  abdominal  segments  are  usually  unfused. 

309.  The  Appendages  also  differ  much  in  form  in  the 
various  representatives  and  on  different  segments  of  the  same 
individual.     This  diversity  of  structure  is  closely  connected 
with  the  variety  of  work  to  be  done,  and  is  an  excellent  illus- 
tration of  the  differentiation  which  accompanies  "  division  of 
labor."     They  are  unquestionably  serially  homologous  organs 
as  is  shown  by  their  similarity  of  origin  and  by  the  funda- 
mental likeness  of  structure, — clearly  to  be  seen  in  the  primi- 
tive forms.     They  may  be  said  to  consist  typically  of  a  basal 


276  •      ZOOLOGY. 

portion  with  one  or  more  Segments,  supporting  two  jointed 
branches, — a  median  and  an  external.  Appendages  may  be 
entirely  wanting  (as  in  the  abdominal  segments J^  insects)  ; 
and  yet  these  may  appear  in  a  rudimentary  form nm  the  early 
stages  of  the  embryo,  only  to  disappear  later.  Where  the 
metamerism  is  obscured  by  fusion,  the  number  of  appendages 
may  be  the  only  indication  we  have  of  the  number  of  segments : 
but  as  we  have  seen,  the  appendages  themselves  are  some- 
times aborted  in  regions  where  they  are  no  longer  needed. 
General  groups  of  appendages  are  as  follows:  (i)  pre- 
oral,  mostly  sensory, — as  antennae;  (2)  oral,  biting  and  suck- 
ing structures, — mandible  and  maxillae;  (3)  thoracic,  chiefly 
walking  appendages;  (4)  abdominal,  variously  modified  (as 
swimmerets,  gills,  etc.)  or  wanting.  The  wings  are  not  to  be 
regarded  as  homologous  with  the  jointed  appendages.  They 
originate  as  expansions  of  the  integument  of  the  body,  sup- 
ported by  numerous  tubular  ribs  or  "  veins "  containing 
branches  of  the  blood-vessels,  tracheae,  and  nerves.  Wings, 
when  present,  comprise  one  (flies)  or  two  pairs  (bees).  Often 
the  anterior  pair  is  hardened  and  serves  merely  as  a  protec- 
tion for  the  second  pair.  Either  pair,  more  often  .the  second, 
may  be  aborted. 

310.  Coelom. — The    development    of    the    arthropods    shows    that    the 
spaces  in  the  body  are  not  truly  ccelomic  as  a  rule,  but  are,  so  to  speak, 
much  enlarged  blood  spaces  containing  the  corpuscle-bearing  fluid.     The 
pericardial   sinus   is  one  of  these.     Such   a  body   cavity  is  known   as   a 
hamoc&le. 

311.  Integumentary    Structures.— The    arthropod    skin 
has  an  epidermal  layer  of  cells  which  secretes  the  chitinous 
cuticle  constituting  the  external  skeleton.     The   chitin  may 
be  mixed  with  salts  of  lime.     Beneath  the  epidermis  is  a  layer 
of  connective  tissue, — the  dermis,  containing  nerves  and  blood 
vessels.     Still  within  these  are  the  longitudinal  muscles  of  the 
body  wall.     When  the  secreted  shell  becomes  thick  and  hard, 
further  growth  is  necessarily  more  difficult.     This  difficulty  is 
usually   overcome   by   moulting,    in   which    process    the    old 


ARTHROPODA.  277 

cuticle  is  separated  from  the  epidermis,  rupturing  along  some 
line  of  weakness,  and  allowing  the  escape  of  the  animal.  This 
moulting  extends  not  only  to  the  minutest  of  the  external 
organs,  but  to  the  stomodcTum  and  proctodseum  as  well.  A 
new  cuticle  begins  to  be  secreted  at  once,  but  this  "  soft- 
shelled  "  condition  is  one  of  great  danger  and  helplessness  to 
the  animal.  The  process  besides  is  exhausting,  and  to  these 
facts  we  may  attribute,  in  part  at  least,  the  small  size  of  most 
arthropods.  The  cuticula  is  laid  down  very  thinly  at  the  joints. 
Thus  is  secured  the  flexibility  necessary  in  locomotion. 

312.  The  Muscles  are  well  developed,  and  many  of  the 
arthropods  are  very  powerful  in  proportion  to  their  size.    The 
circular  muscles  characteristic  of  the  annulata  are  lacking  in 
the  arthropods.     The  chief  body  muscles  are  the  longitudinal 
which  cause  the  flexion  and  extension  of  the  segments.    There 
are   in   addition  the  muscles   by  which   the  appendages   are 
moved.      These   fibres   are   of   the   cross-striate   type.      Less 
massive  groups  of  fibres  are  found  in  the  walls  of  portions  of 
the  digestive  tract. 

313.  The    Digestive    Organs. — The    alimentary    tube    is 
typically  rather  complex.     It  commences  with  a  mouth  which 
is   usually   supplied   with   three   or   more   pairs   of   external 
appendages  assisting  in  the  capture,  transfer,  and  preparation 
of  food.     This  is  followed  by  an  cesophagus  either  with  or 
without  a  crop;  a  stomach  frequently  consisting  of  several 
regions:   viz.    (a)    a  proventrlculus  or  gizzard,   and    (b)    a 
ventriculus  or  stomach  proper ;  an  intestine  which  is  not  always 
clearly  marked  off  from  the  stomach;  and  a  posterior  open- 
ing,— the   anus.      The   development  of   the  gut  shows   both 
stomodseum  and  proctodaeum  (see  §  90).    The  former  is  often 
very  extensive, — embracing  even  the  proventriculus,  in  which 
chitinous  grinding  plates  may  occur   (cray-fish,  cockroach). 
The  "  salivary  "  glands  when  present  open  into  the  cesophagus 
or  mouth  cavity.     Into  the  mesenteron  important  digestive 
glands  may  open,  as  the  pyloric  cseca  (many  insects),  or  liver 


278  ZOOLOGY. 

(cray-fish  and  spiders).  The  Malpighian  tubules  (see  dissec- 
tion of  the  grasshopper)  associated  with  the  hind  gut  are 
believed  to  be  excretory  rather  than  digestive  in  function. 
The  digestive  system  as  a  whole  is  strikingly  correlated  with 
the  character  of  food  used,  which  is  exceedingly  diversified 
in  this  phylum.  This  can  be  appreciated  only  by  extended 
observation  and  comparison.  The  student  is  urged  to  com- 
pare such  figures  of  these  organs  as  he  may  be  able  to  find  in 
the  reference  texts  at  his  command. 

314.  Respiration. — In  some  instances  the  Arthropoda  ob- 
tain their  oxygen  directly  from  the  air,  in  others  from  the 
water.  In  the  latter  the  exchange  is  effected  through  the  body 
wall,  or  by  gills.  These  are  essentially  thin  outgrowths  t)f 
the  body  wall,  with  the  cuticula  much  reduced  or  absent, 
into  which  the  blood  passes  (e.  g.,  the  majority  of  Crustacea). 
In  the  former  it  takes  place  wholly  by  means  of  tubular  air- 
passages  or  trachea  (insects),  or  these  may  be  supplemented  by 
thinned  folds  of  the  body  wall — book-lungs  (spiders).  By 
these  devices  the  oxygen  of  the  air  or  water  and  the  blood 
are  brought  into  intimate  relations.  In  the  water-breathing 
forms  the  gills  are  either  the  modified  appendages  (Limulus, 
Asellus),  or  specialized  outgrowths  from  them  or  from  the 
general  body  wall  (cray-fish)  (Fig.  124,  g).  The  gills 
vary  widely  in  number  and  position,  but  are  found  especially 
in  connection  with  the  thoracic  and  abdominal  appendages. 
The  air-breathing  forms  possess  a  system  of  interbranching 
tubes  which  may  open  to  the  exterior  by  a  pair  of  stigmata  or 
pores  in  each  somite.  These  tubes  unite,  branch  and  penetrate 
to  every  portion  of  the  body.  The  air  is  carried  to  the  blood 
rather  than  the  blood  to  the  air.  The  tubes  are  lined  by  a 
thin  layer  of  cuticle,  and  are  kept  open  by  a  spiral  thread  of 
the  same  material  reinforcing  the  wall.  The  book-lungs  when 
present  lie  within  a  sac  which  opens  to  the  exterior  by  a  stigma 
or  pore,  and  consist  of  a  series  of  plaitings,  within  which  the 
blood  circulates  and  between  which  the  air  circulates. 

The  larvae,  especially  of  air  breathers,  are^often  developed 


ARTHROPODA. 


279 


in  conditions  very  different  from  those  chosen  by  the  adults. 
This  fact  may  make  necessary  very  important  changes  in  the 
respiratory  organs  in  the  metamorphosis.  Some  forms  are 
even  water  breathers  in  the  larval  stage  and  air  breathers  in 

the  adult  (dragon-flies). 

FIG.  124. 


FIG.  124.  Diagrammatic  cross-section  of  Cray-fish  in  the  thoracic  region,  to  *how 
relation  of  circulation  and  respiration,  a,  appendage;  c,  carapace;  c.f.,  flap  of  cara- 
pace overhanging  the  gills;  d,  digestive  tube;  g,  gill;  h,  heart;  I,  liver;  m,  body  muscles; 
m',  muscles  of  the  appendages;  n.c.,  nerve  cord;  p.s.,  pericardial  sinus;  r,  reproductive 
glands;  st,  sternal  artery;  v.a.,  ventral  artery;  v.s.,  ventral  blood  sinus  in  which  the 
nerve  cord  lies.  Modified,  from  Lang. 

Questions  on  the  figure. — What  is  the  relation  of  the  gills  to  the  body 
•wall?  Follow, the  course  of  the  circulation  by  the  arrows.  It  leaves  the 
heart  by  definite  arteries  and  comes  back  by  less  definite  blood  sinuses. 
What  is  the  function  of  the  valves?  What  gain  is  there  in  the  position 
of  the  ventral  nerve  cord  in  the  blood  sinus? 

315.  Circulation. — The  heart  or  pulsating  organ  when 
present  is  dorsal  and  may  be  much  elongated,  with  an  enlarge- 
ment in  each  somite.  It  lies  in  a  membrane-bounded  cavity 
called  the  pericardial  sinus  (Fig.  124,  ps.),  which  is  a  part  of 
the  hsemoccele  or  secondary  body  cavity  (§  310).  The  blood 
comes  to  the  pericardial  cavity  and  enters  the  heart  by  means  of 


280  ZOOLOGY. 

slit-like  openings,  with  valves.  Definite  arterial  vessels  leave 
the  heart  and  pass  to  capillary  regions  and  thereupon  open 
into  irregular  spaces  in  the  tissues  without  definite  walls 
(lacunae}.  The  haemoccele  is  in  reality  an  enlarged  lacuna. 
In  insects  there  is  an  anterior  artery  only ;  in  spiders  and  crus- 
tacea,  posterior  and  lateral  arteries  also  occur.  The  return  of 
the  blood  takes  place  through  the  irregular  hsemoccele  spaces 
(lacunae).  These  become  more  definite  in  form  as  they  near 
the  pericardial  chamber,  or  as  they  approach  the  gills  in 
aquatic  forms.  One  of  the  more  important  blood  spaces  is 
the  ventral,  in  which  the  nerve  cord  lies  (Fig.  124,  v.s.).  The 
blood  corpuscles  are  colorless  and  amoeboid.  The  plasma  may 
be  variously  colored  by  pigments  which  seem  to  assist  in  the 
work  of  respiration. 

316.  Excretion. — The   importance   of   excretion   increases 
with  the  activity  of  animals.     Except  in  Peripatus  it  is  not 
conclusive   that  any   of   the   adult   excretory   organs   in  this 
phylum  are  homologous  with  the  segmental  organs  of  Annu- 
lata.     In  insects  and  spiders  there  are  excretory  tubules  com- 
municating with  the  hind  gut.     In  the  cray-fish  and  related 
forms  a  pair  of  excretory  glands — "  green  glands  " — open  at 
the  base  of  the  antennae.     It  is  of  importance  to  remember 
that  the  exoskeleton  of  the  Arthropoda  is  an  excretion,  which 
is  incidentally  protective  and  supportive. 

317.  The  Nervous  System  consists  essentially  of  the  same 
parts  as  have  been  described  for  the  annelids.     It  is,  however, 
on  the  whole,  more  fully  developed.    This  development  accords 
with  the  differentiation  which  we  have  seen  in  the  somites  and 
body  regions.     The  brain  and  sub-cesophageal  ganglia,   for 
example,  have  become  more  pronounced  with  the  differentia- 
tion of  the  head;  accompanying  the  fusion  of  the  body  seg- 
ments there  is  a  massing  of  the  corresponding  ganglia;  and 
in  general,  everything  considered,  those  ganglia  are  best  de- 
veloped which  lie  in  the  best  developed  somites.  The  concen- 
tration of  the  ganglia  of  the  ventral  cord  may  continue  until 


ARTHROPODA.  28 1 

they  form  practically  one  mass.  Nerves  arise  from  the  brain, 
from  the  connective  about  the  gullet,  and  from  the  ventral 
ganglia. 

318.  Organs  of  Special  Sense. — As  the  thickened  cuti- 
cular  covering  of  the  arthropods  develops,  it  is  apparent  that 
much  of  the  sensitiveness  of  the  surface  to  external  conditions 
must  be  lost  unless  special  structures  are  produced  to  com- 
pensate for  this  by  the  reestablishment  of  connection  between 
the  internal  organs  and  the  outside  world.  Such  structures 
we  find  in  the  chitinous  hairs  of  various  shapes  which  project 
beyond  the  surface  and  in  pits  or  canals  which  pierce  the 
skeleton.  These  all  have  nervous  connections  and  have  been 
variously  interpreted  as  tactile,  taste,  auditory,  and  olfactory 
organs.  They  are  especially  abundant  in  the  more  movable 
portions,  particularly  those  about  the  mouth.  Figures  illus- 
trating the  great  variety  of  forms  of  such  hairs  should  be 
sought  in  the  reference  texts. 

At  least  three  classes  of  organs  have  been  described  as 
auditory  among  the  arthropods:  (a)  vibratile  hairs,  as  in  the 
case  of  the  male  mosquito  (Fig.  40)  ;  (&)  otocysts,  as  in  many 
aquatic  forms;  (c)  a  tympanum  or  membrane  in  connection 
with  which  are  special  nervous  -cells  for  the  reception  of  the 
vibrations  (as  in  the  grasshopper  and  other  insects).  The 
otocysts  of  the  Crustacea  may  be  open  or  entirely  closed.  In 
the  former  case  the  animal  itself  may  place  the  otoliths  in  the 
otocyst  in  the  form  of  grains  of  sand.  Recent  investigations, 
however,  tend  to  show  that  the  function  of  this  organ  is  not 
hearing,  so  much  as  that  of  informing  the  animal  of  its  rela- 
tion to  the  pull  exerted  by  gravity,  thus  enabling  it  to  keep 
its  equilibrium. 

There  are  two  classes  of  eyes  in  the  group:  (a)  compound 
eyes,  made  up  of  numerous  similar  elements,  as  in  the  insects 
and  Crustacea,  and  (&)  simple  eyes — ocelli — found  alone  in 
spiders  and  in  many  larvae,  or  in  connection  with  the  com- 
pound eyes,  as  in  many  insects. 


282 


ZOOLOGY. 


The  compound  eye  is  made  up  of  elements   (ommatidia) 
radially  arranged  about  the  end  of  the  optic  nerve.     Each 


FIG.  125. 


ommatidium  is  probably  capable  of  forming 
an  image  of  a  limited  portion  of  the  field, 
and  consists  of  (i)  a  cuticular  cornea,  ap- 
pearing externally  as  a  "facet,"  (2)  a  cel- 
lular lens  or  cone  which  directs  the  rays  of 
light,  (3)  sensory  retinal  cells  which  re- 
ceive the  light,  and  (4)  pigment  cells  which 
separate  the  retinal  elements  of  adjacent 
ommatidia,  and  play  an  important,  though 
not  fully  understood,  role  in  vision  (see 
Figs.  42  and  125). 

FIG.  125.  An  ommatidium  or  eye-element  from  the  eye  of 
the  Lobster  (after  G.  H.  Parker),  c,  cornea  (cuticle);  c.h., 
corneal  hypodermis,  which  secretes  the  cuticle;  co.,  cone  cells; 
cr.,  crystalline  cone;  n,  nuclei;  n.f.,  nerve  fibres;  r.d.,  distal 
or  outer  retinula  cells;  r.p.,  proximal  or  inner  retinula  cells; 
rJi.,  rhabdome. 

Questions  on  the  figure. — Identify  the  following 
regions:  (i)  protecting  part  including  the  cornea 
and  hypodermal  cells;  (2)  focussing  portion, — the 
crystalline  cone  and  the  cone  cells ;  (3)  the  pigmental 
elements  of  the  retina  (distal  and  proximal  retinular 
cells)  the  former  of  which  prevent  rays  of  light 
entering  one  ommatidium  from  passing  obliquely 
into  adjacent  ones;  the  proximal  cells  may  be  more 
immediately  connected  with  (4)  the  nervous  elements 
which  unite  the  eye  with  the  nerve  centres.  Define 
an  ommatidium.  Is  it  known  whether  the  image  is 
inverted  in  such  an  eye  as  this? 

319.  Library  Exercise. — If  time  allows  some  student  might  be  required 
to  make  a  more  detailed  report  of  the  structure  of  the  compound  eye  in 
Arthropods  and  its  method  of  image  formation.     Other   reports  may  be 
made,  in  which  drawings  of  the  various  sense-organs  in  arthropods  are 
presented  to  the  class,  especially  the  various  types  of  auditory  organs. 

320.  Reproduction  and  the  Reproductive  Organs. — Re- 
production in  Arthropods  is  sexual.    With  few  exceptions  the 
sexes  are  permanently  separate.     There  is  often  much  differ- 
ence in  the  size,  color,  structure,  and  activity  of  the  two  sexes. 


ARTHROPODA.  28 


The  males  are  often  smaller,  more  active,  and  more  highly 
colored  than  the  females  (see  "sexual  dimorphism,"  §  145). 
Sometimes  the  members  of  a  single  sex  are  dimorphic,  as  in 
the  workers  and  queens  among  the  bees.  This  is  correlated 
with  individual  division  of  labor  in  the  social  insects. 

The  sexual  organs  are  usually  paired,  and  in  the  female 
consist  of  the  ovaries  (which  maybe  subdivided  into  ovarioles) , 
oviducts,  receptacula  seminis  in  which  spermatozoa  are  stored 
at  copulation,  accessory  glands,  sometimes  external  copulating 
and  egg-depositing  organs.  The  male  has  testes;  vasa  de- 
ferentia,  which  may  have  special  enlargements  for  the  storing 
of  spermatozoa  and  the  formation  of  sperm  masses;  and  ex- 
ternal copulatory  organs.  See  figures  of  the  sexual  organs 
of  the  honey-bee  or  other  representative  insect  in  the  reference 
texts.  Compare  them  with  those  of  the  snail. 

321.  Parthenogenesis. — In  several  insect  types  the  eggs 
have  the  power  of  developing  without  being  fertilized  by  the 
male  element.     Its  occurrence  is  determined  primarily  by  the 
absence  of  males,  but  even  when  males  are  present  the  female 
may  often  deposit  unfertilized  eggs.     She  is  influenced  to  do 
this  possibly  by  the  special  conditions  of  temperature,  nutri- 
tion, and  the  like,  to  which  she  is  subject. 

The  individuals  resulting  from  parthenogenesis  may  differ 
very  materially  from  those  produced  by  the  normal  sexual 
method.  In  the  case  of  the  bee,  the  males  or  drones  come  from 
unfertilized  eggs,  and  the  workers 'and  queens  from  fertilized. 
The  cause  of  the  differences  between  workers  and  queens  is 
apparently  one  of  nutrition  purely.  Biologically,  partheno- 
genesis is  to  be  considered  as  a  modified  or  abbreviated  form 
of  sexual  reproduction,  in  which  the  stimulus  to  cleavage 
comes  from  some  source  other  than  the  male  cell. 

322.  Development. — After  fertilization  the  nucleus  divides 
as  described  for  other  forms,  but  usually,  on  account  of  the 
abundant  yolk  which  the  eggs  contain,  complete  segmentation 
of  the  cell  is  not  effected.    After  a  series  of  divisions  some  of 


284  ZOOLOGY. 

the  nuclei  assume  a  superficial  position  where  they  become 
surrounded  by  protoplasm,  and  form  the  blastoderm  (Fig.  n, 
D,  j).  This  is  described  as  partial  and  peripheral  segmenta- 
tion. On  the  sicje  of  the  egg  where  the  embryo  is  to  lie,  a 
thickening  called  the  ventral  plate  is  formed.  From  this  area 
of  the  blastoderm  there  arises,  by  specialization,  by  insinking, 
and  by  multiplication  of  the  cells,  the  three-layered  condition. 
The  presence  of  yolk  so  obscures  and  complicates  the  process 
that  the  student  must  be  referred  to  more  comprehensive  books 
for  even  an  outline  of  it. 

323.  The  Later  Development  may  be  either  direct  or  in- 
direct. That  is  to  say,  the  young  when  hatched  may  be  the 
adult  in  miniature,  possessing  its  form  and  habits,  or  may 
have  a  very  different  form  and  assume  the  adult  condition  by 
one  or  more  metamorphoses.  The  differences  between  the 
larval  and  adult  conditions  may  be  slight  or  very  great.  To 
effect  the  change  from  larva  to  adult  a  series  of  moultings  of 


FIG.  126. 


FIG.   126.     The  Zoea  of  Cancer  irroratus.     From  Verrill.     x   15. 

Questions  on  the  figure* — Compare  with  the  larva  of  lobster  (Fig.  129) 
and  with  the  Megalops  (Fig.-  127),  and  note  likenesses  and  differences. 


ARTHROPODA. 


the  chitinous  covering  is  usually  necessary;  these  may  be  ac- 
companied or  preceded  by  periods  of  rest,  in  which  important 
internal  changes  take  place.  The  metamorphosis  is  more  com- 
mon among  insects  (Figs.  140  and  146),  although  a  similar 

FIG.  127. 


FIG.   127.     Megalops  of  Cancer  irroratus.     From  Verrill.     x  15. 

Questions  on  the  figure. — Compare  with  Figs.  126  and  129,  and  make 
note  of  the  chief  points  of  contrast.  Compare  also  with  adult  crab  (Fig. 
128).  What  differences  are  to  be  noted  between  the  development  in  the 
lobster  and  in  crabs?  Is  the  larval  or  adult  crab  more  like  the  lobster? 

thing  happens  in  many  of  the  Crustacea  (as  crabs,  Figs.  126 
to  128).  In  spiders  the  development  is  direct.  The  eggs  of 
many  insects  hatch  as  worm-like  larvae  (grubs,  maggots, 
caterpillars).  These  are  usually  active,  voracious,  fat-storing 
animals,  which  after  a  period  pass  into  a  resting  condition, 
often  surrounding  themselves  with  protective  coverings 
(cocoons).  During  this  quiescent  stage  they  are  described  as 
pupa.  In  the  pupal  stage  the  accumulated  fat  is  used  by  the 
organism  in  forming  the  new  organs  of  the  adult  or  imago. 
The  internal  larval  organs  may  be  torn  down  completely  by 


286 


ZOOLOGY. 


the  aid  of  amoeboid  cells  and  be  made  to  contribute  material 
to  rebuild  the  new.  The  extent  of  these  changes  can  only 
be  realized  by  a  comparison  of  the  structure  of  a  caterpillar 
and  of  the  butterfly  into  which  it  develops.  The  larvae  may 
be  suited  to  aquatic  life,  the  adult  to  aerial;  the  larva  may  be 
carnivorous  or  herbivorous,  the  adult  may  live  on  the  nectar 
of  flowers.  These  changes  of  habit  are  closely  correlated  with 
the  changes  of  structure  noted  in  the  metamorphosis.  The 
reproductive  organs  are  not  mature  until  the  imago-stage  is 
reached.  Frequently  the  imago  only  survives  long  enough 
to  insure  the  laying  of  fertilized  eggs. 

FIG.  128. 


FIG.   128.     Violet   Land-Crab.     After   Shufeldt. 

Questions  on  the  figure. — Compare  the  crab  with  the  lobster  (Fig. 
130)  as  to  the  development  of  the  body-regions,  segmentation,  appendages, 
etc.  Compare  all  the  figures  of  crabs  available  and  note  in  what  respects 
they  vary  externally. 

324.  Library    References. — Make   a   report  on  the   metamorphosis   in 
Crustacea.    What  is  meant  by  an  incomplete  metamorphosis?    Illustrations. 

325.  Ecology. — When  we  remember  the  great  number  of 
species  and  of  individuals  in  the  group  of  arthropods  we  are 
forced  to  realize  something  of  their  importance  in  their  rela- 
tion to  other  forms  of  life  on  the  earth.     Their  numbers  and 
their  enormous  power  of  reproduction  make  it  inevitable  that 


ARTHROPODA.  287 

they  become  pests  and  threaten  the  existence  of  the  plants  and 
animals  on  which  they  prey,  and  likewise  that  they  become 
important  elements  in  the  food  supply  of  animals  which  prey 
on  them.  It  is  only  by  their  great  reproductive  power  that 
they  can  hold  their  own  against  their  many  enemies, — the  birds 
and  other  insectivorous  animals,  and  the  accidents  of  climate, 
etc. 

From  a  human  point  of  view  they  may  be  the  greatest  pests 
or  the  greatest  helpers.  In  the  voracious  larval  stage  they 
devour  waste  material  as  scavengers,  strip  vegetation,  spread 
disease,  produce  silk,  and  furnish  food  to  the  higher  animals. 
In  the  adult  stage  they  may  destroy  crops;  cross-fertilize 
flowers  in  their  search  for  nectar,  which  they  may  store  for 
themselves  and  their  young — to  be  intercepted  in  the  case  of 
the  bee  by  man ;  may  spread  contagious  diseases ;  may  devour 
stored  grain  or  by  their  mere  presence  become  a  nuisance  to 
man  and  the  domestic  animals.  In  both  stages  they  may  be 
parasites  on  man  and  other  animals.  Few  of  the  arthropods 
are  directly  useful  as  food  to  man,  though  lobsters,  cray-fish, 
shrimps,  etc.,  are  important  items  in  our  food  supply.  Many 
special  devices  of  structure  and  of  instinct  have  arisen  making 
their  continued  existence  in  the  presence  of  their  enemies 
possible.  Indeed  there  is  no  group  of  animals  in  which  so 
many  and  such  interesting  adaptations  to  the  special  conditions 
of  life  are  found  as  among  the  arthropods.  All  are  provided 
with  some  degree  of  external  protective  covering.  Many  are 
so  colored  and  shaped  as  to  be  inconspicuous  in  their  natural 
environment.  Some  are  endowed  with  offensive  odor  and 
taste,  some  with  stinging  organs.  Others  which  are  them- 
selves perfectly  harmless  are  so  much  like  forms  which  are 
repulsive  or  dangerous,  as  to  be  preserved  thereby  from  their 
enemies  (see  Chapter  VIII). 

Many  insects  as  ants,  bees,  and  wasps  are  strikingly  social 
in  their  habits,  and  show  a  high  degree  of  differentiation 
among  themselves.    Among  the  bees  a  special  class  of  females  ' 
— the  queens — lay  the  fertilized  eggs,  the  other  females — the 


288  ZOOLOGY. 

workers — being  sexually  immature.  In  the  ants,  still  further 
division  of  labor  occurs  among  the  workers.  Some  individuals 
act  as  soldiers  for  the  protection  of  the  ordinary  workers. 
Some  species  of  ants  make  slaves  of  other  species  of  ants 
which  do  the  work  of  the  colony,  or  of  other  animals  (aphides) 
for  the  purpose  of  feeding  on  their  secretions.  A  high  order 
of  intelligence  and  skill  is  shown  by  certain  members  of  this 
group, — the  highest,  apparently,  shown  by  any  of  the  inverte- 
brate animals. 

326.  Library  Exercises. — Reports  on  the  social  life  of  bees  and  ants; 
on  the  animals  captured  and  utilized  by  ants ;  on  power  of  flight  in  ants ; 
on  queens  among  ants  and  bees ;  on  myrmecophilous   (ant-loving)   insects ; 
on  intelligence  among  insects  and  spiders. 

327.  Classification. 

Cla.is  I.  Crustacea  (Crayfish,  Crabs,  Barnacles,  etc.). — Arthropoda,- 
chiefly  inhabiting  the  water  and  breathing  by  means  of  gills  or  through 
the  body  wall.  The  head  typically  consists  of  five  segments  more  or  less 
fused  and  bearing  two  pairs  of  antennae  or  feelers,  one  pair  of  mandibles, 
and  two  pairs  of  maxillae.  The  thorax  or  second  region  of  the  body  may 
be  separate  from  or  fused  with  the  head  (ccphalo thorax}.  It  possesses  a 
variable  number  of  segments,  which  usually  bear  the  locomotor  append- 
ages. The  remainder  of  the  body  (abdominal  segments)  is  normally  of 
distinct  segments  in  which  the  appendages  are  much  reduced.  The  chitin- 
ous  skeleton  is  ordinarily  well  developed. 

Subclass  i.  Entomostraca. — Crustacea,  small  and  simple  in  organization, 
with  a  variable  number  of  segments  of  which  the  appendages  are  simple 
and  less  diverse  than  in  the  next  subclass.  Many  of  them  are  parasitic 
and  degenerate.  A  metamorphosis  occurs.  The  group  embraces  many 
small  free  forms,  found  both  n  fresh  and  salt  water,  some  fish  parasites, 
and  the  sedentary  barnacles.  Here  belong  Cyclops  and  Daphnia,  which 
occur  abundantly  in  fresh-water  pools  and  feed  on  the  algae  common 
there.  They  constitute  an  important  portion  of  the  food  of  the  fresh- 
water fishes.  They  multiply  very  rapidly  and  keep  closely  up  to  the  limit 
of  the  food  supply.  The  eggs  of  many  of  them  can  resist  drying  to  a 
remarkable  d--  ^ree.  This  is  of  manifest  importance  when  we  remember 
that  they  frequent  pools  in  which  drouth  is  not  uncommon. 

The  barnacles  (Cirripedia)  are  Crustacea  which  in  adult  life  become 
attached  to  the  rocks  near  low  water-mark  or  to  floating  objects  of 
various  kinds.  The  bottoms  of  ships  become  foul  with  them.  The  group 
is  especially  interesting  in  that  it  points  to  the  giving  up  of  free  motion, 
which  its  ancestors  possessed,  for  a  mode  of  life  wholly  different,  and 
demanding  marked  changes  of  structure.  They  possess,  besides  the 


ARTHROPODA.  289 

organs  for  attaching  themselves,  bivalve  shells  similar  to  those  of 
Mollusca,  for  protection;  they  are  often  hermaphroditic,  which  is  a  very 
uncommon  thing  in  arthropods.  The  advantages  gained  by  their  special 
habits  are  evident.  The  waters  near  the  shore  contain  a  great  deal  of 
organic  debris,  and  any  organism  which  can  attach  itself  here  and  yet 
be  protected  from  destruction  by  the  waves  is  fortunate.  Those  attached 
to  floating  objects  are  brought,  without  their  effort,  into  constantly  chang- 
ing localities. 

Subclass  2.  Malacostraca. — Crustacea  of  larger  size  and  more  highly 
organized.  Segments,  except  in  one  order,  twenty,  and  well  differentiated. 
Nineteen  of  these  segments  bear  appendages.  The  first  stage  in  the  meta- 
morphosis (the  nauplius')  is  usually  passed  before  hatching.  The  group 

FIG.  129. 


FIG.   129.     Larva  of  Lobster    (Homarus  americanus)    removed  from  egg  shell. 
From  Herrick. 

Questions  on  the  figure. — Compare  with  the  adult  (Fig.  130)  ancr  note 
similarities  and  differences?  Examine  Dr.  Herrick's  figures  (Bull.  U.  S. 
Fish  Commission  for  1895)  and  notice  the  gradual  change  to  the  adult 
condition  by  successive  moultings.  What  structures  can  you  identify? 

embraces  (i)  the  Decapoda,  or  the  lobsters,  crabs,  cray-fishes  and  shrimps, 
which  agree  in  the  possession  of  ten  walki^  g  feet,  eyes  on  movable  stalks 
and  a  carapace  covering  the  thirteen  fused  segments  of  the  cephalothorax ; 
and  (2)  the  Tetradecapoda,  comprising  numerous  smaller  types  such  as 
beach-fleas,  sow-bugs  or  wood-lice,  in  which  head  and  thorax  are  not 
fused,  the  eyes  are  not  movable,  and  the  walking  appendages  are  fourteen. 
The  cray-fish  and  lobsters  have  well-developed  abdominal  segments, 
whereas  in  the  crabs  the  abdomen  is  reduced  and  bent  under  the  thorax, 
which  becomes  broad  and  massive  (Figs.  127,  128).  Thei^arger  Crustacea 
are  omnivorous,  almost  all  organic  matter,  dead  or  living,  being  acceptable. 
Lobsters  are  known  to  attack  and  devour  fishes.  The  lobster  (Homarus, 
Figs.  129  and  130),  of  which  there  are  two  species, — an  American  and  a 
European, — is  economically  the  most  important  member  of  the  group,  and 
stands  next  the  oyster  as  the  most  important  invertebrate  food  species. 
20 


290 


ZOOLOGY. 
FIG.  130. 


FIG.   130.     The   American    Lobster    (Homarus   americanus).     From    Herrick. 

Questions  on  the  figure. — What  body-regions  are  distinguishable  in 
the  lobster?  Compare  by  actual  measurement  the  size  of  the  crushing 
claw  with  that  of  the  body.  How  many  segments  in  the  abdominal  region  ? 
Compare  with  Fig.  128. 

It  is  estimated  that  as  many  as  one  hundred  million  lobsters  have  been 
taken  in  a  single  year  in  New  England  and  Canadian  waters.  There  is  no 
doubt  that  the  lobster  is  in  immediate  danger  of  extinction  as  a  food 
animal,  as  is  shown  by  the  fact  of  greater  difficulty  in  obtaining  them 
and  by  the  decrease  in  the  average  size  of  the  animals  put  on  the  market. 
This  decrease  occurs  in  the  face  of  the  fact  that  the  mature  female  produces 
from  ten  thousand  to  one  hundred  thousand  eggs.  These  are  carried 
under  the  abdomen  of  the  mother  until  hatched,  which  requires  a  period 


ARTHROPODA.  29! 

of  ten  or  eleven  months.  After  hatching  the  young  undergo  a  series  of 
moultings  during  which  time  they  are  the  prey  of  many  kinds  of  enemies. 
Such  is  the  mortality  that  on  an  average,  two  out  of  an  entire  spawning 
do  not  reach  maturity.  Two  general  methods  have  been  tried  to  make 


131- 


FIG.   131.     Palamonetes  -culgaris.     From   Verrill. 

Questions  on  the  figure. — Compare  the  appendages  of  Palsemonetes 
with  those  of  the  lobster,  the  crab  and  Gammarus.  What  seem  to  be  the 
functions  of  the  various  appendages,  so  far  as  position  and  form  may 
indicate? 

FIG.  132. 


FIG.   132.     Gammarus  ornatus.     From  Verrill. 

Questions  on  the  figure. — How  does  this  form  compare  with  the 
lobster  and  the  crabs  in  differentiation  of  the  segments,  in  fusion  of  the 
segments  and  in  the  differentiation  of  the  appendages? 

good  the  decline  in  the  supply:  first,  legislation  forbidding  the  taking  of 
animals  under  the  size  which  indicates  sexual  maturity  (eight  to  twelve 
inches),  and  forbidding  the  capture  of  females  carrying  the  developing 
embryos;  and,  second,  attempts  on  the  part  of  the  national  government  to 
hatch  artificially  and  care  for  the  moulting  young  under  such  conditions 
that  they  will  be  protected  from  their  natural  enemies.  The  problem  is 


ZOOLOGY. 


not  yet  solved,  and  in  the  meantime  another  source  of  food  is  likely  to  be 
destroyed  through  overfishing. 

The  cray-fish  is  prized  for  food  in  European  countries,  but  is  little  used 
in  America  as  yet.  Shrimps,  prawns,  the  "  soft-shelled  "  or  blue  crab  are 
all  of  considerable  importance  in  this  regard.  The  smaller  Crustacea  are 

FIG.   133- 


FIG.    133.     Caprella  geometrica.     From  Verrlll.     x  4. 

Questions  on  the  figure. — In  comparison  with  other  Crustacea  what 
are  the  aberrant  or  peculiar  features  of  this  form?  See  also  figures  in 
reference  texts  (e.  g.,  Parker  and  Haswell's  Zoology,  Vol.  I,  p.  546). 

a  very  important  element  in  the  food  supply  of  the  fishes,  both  in  the  fresh 
waters  and  in  the  sea. 

Class  II.  Onychophora. — This  class  of  arthropods  includes  only  the 
one  genus,  Peripatus,  which  is  interesting  chiefly  because  it  is,  in  some 
degree,  intermediate  between  the  Annulata  and  the  higher  arthropods. 
There  are  about  a  dozen  species  of  Peripatus,  chiefly  from  Africa  and 
South  America.  They  are  found  in  moist  places,  under  wood,  stones,  and 
in  rotting  bark.  They  agree  with  the  chaetopod  annulates  (see  270)  in  the 
possession  of  segmental  organs  (nephridia),  a  dermo-muscular  sac, -and 
poorly  developed  appendages.  The  segments  are  also  hdmonomous  (see 

FIG.  134- 


FIG.   134.     Peripatus  capensis.     From   Nicholson   after   Moseley. 

Questions  on  the  figure. — Externally  in  what  respects  is  this  form  like 
the  Annelids?  In  what  respects  different  from  them?  Of  what  special 
zoological  interest  is  this  genus?  What  are  its  habits?  In  what  respects  is 
it  like  and  in  what  unlike  the  centipede  (Fig.  135)  ? 

§  258)  as  in  the  worms.  The  relationship  to  arthropods  is  indicated  by  the 
possession  of  tracheae,  by  the  substitution  of  haemoccele  (the  enlarged 
lacunae  in  which  circulation  occurs)  for  the  true  ccelom,  and  by  the 
differentiation  of  some  of  the  anterior  segmental  appendages  as  mouth 
parts.  The  Onychophora  resemble  the  larval  condition  of  those  insects 
which  undergo  a  metamorphosis  much  more  than  the  adult  stages.  This 


ARTHROPODA.  293 

suggests  that  they  are  more  closely  related  to  the  ancestral  types  from 
which  the  insects  have  sprung  than  to  the  insects  themselves  (Fig.  134). 
Class  III.  Myriapoda  (Centipedes,  etc.)- — Tracheate  arthropods  with 
a  worm-like  body.  Segments  numerous,  and  much  alike,  one  (or,  in 
Diplopoda,  two)  pair  of  appendages  to  each  segment.  The  head  is  dis- 
tinct and  bears  antennae  and  mouth  parts.  The  eyes  are  numerous  and 
simple  (ocelli}.  In  fundamental  structure  and  development  the  myriapods 

FIG.  135. 


FIG.    135.     Centipede  (Scolopendra  heros).     Photo  by  Folsom.     Four-fifths  natural  size. 

Questions  on  the  figure. — What  differentiation  of  segments  is  ap- 
parent? Are  there  any  fusions  into  body-regions?  What  is  the  law  of 
the  occurrence  of  appendages?  What  diversity  is  there  among  them? 

resemble  insects.  There  are  two  principal  orders.  One  embraces  the 
centipedes  which  are  carnivorous,  have  biting  jaws,  have  one  pair  of 
appendages  to  each  segment,  and  are  poisonous.  The  second  includes  the 
millipedes  which  are  vegetable  feeders  and  possess  mandibles  suited  to 
chewing  vegetable  matter.  They  are  wholly  harmless.  They  have  two 
pairs  of  legs  to  each  of  the  numerous  segments  except  the  first  four.  Both 
centipedes  and  millipedes  inhabit  the  land,  and  frequent  dark  places.  Many 
are  nocturnal  in  habit  (Fig.  135). 

Class  IV.  Hexapoda  (Insects). — Tracheate  arthropods  with  three  dis- 
tinct body  regions, — head,  thorax,  and  abdomen.  The  head  has  four  seg- 
ments with  appendages, — a  pair  of  antennae  and  three  pairs  of  mouth  parts. 
The  thorax  has  three  segments  (pro-,  meso-  and  meta-thorax),  each  of 
which  bears  a  pair  of  legs ;  the  meso-thorax  and  the  meta-thorax  may  each 
bear  a  pair  of  wings.  The  abdomen  has  a  variable  (often  obscure) 
number  of  segments.  Its  appendages  are  usually  entirely  wanting  or  much 
reduced.  A  metamorphosis  frequently  occurs.  The  larval  condition  often 
suggests  the  annelids  and  the  myriapods  in  the  similarity  of  its  segments, 
and  in  the  numerous  appendages. 

The  student  is  referred  to  more  comprehensive  works  for  an  exposi- 
tion of  the  numerous  orders  of  this  enormous  group  of  Hexapoda.  Only 
the  more  important  are  suggested  below. 

Order  Aptera    (without  wings). — This   order  embraces   a  number  of 


294 


ZOOLOGY. 


minute,  wingless  insects  which  do  not  undergo  any  metamorphosis.  The 
body  is  covered  with  scales  or  hairs.  The  spring-tails  and  snow-fleas  are 
examples.  These  make  their  leaps  by  suddenly  straightening  out  a  tail- 
like  structure  which  is  bent  under  the  body  when  at  rest.  They  are  not 
the  only  wingless  insects  and  hence  the  name  is  somewhat  misleading. 
See  Fig.  1.36. 

FIG.  136. 


FIG.   136.     Campodea, — a  Thysanuran.     Magnified  30  times.     By  J.  W.   Folsom. 

Questions  on  the  figure.— -In  what  respects  does  this  form  seem  inter- 
mediate between  the  Myriapods  and  the  higher  insects?  How  does  this 
compare  with  the  larvae  of  insects  which  undergo  a  metamorphosis?  Can 
you  distinguish  head,  thorax,  and  abdomen? 


Order  Orthoptera  (straight  wings'). — In  this  order  the  metamorphosis 
is  incomplete  or  lacking.  There  are  usually  two  pairs  of  wings,  the  an- 
terior often  somewhat  thickened,  serving  as  a  cover  for  the  posterior. 
Mouth  parts  are  adapted  to  biting  and  chewing.  Here  belong  the  cock- 
roaches, grasshoppers,  crickets,  locusts,  katydids,  walking-stick  insect. 
The  order  is  of  considerable  economic  importance.  Most  of  its  members 
are  vegetable  feeders  and  when  they  are  gregarious  are  often  very  de- 


ARTHROPODA. 


295 


structive.  The  Rocky  Mountain  locust,  so  named  because  it  breeds  on  the 
plateau  at  the  eastern  base  of  these  mountains,  in  1873  and  again  in  1878, 
migrated  eastward  over  Nebraska  and  Kansas  in  search  of  food,  literally 
stripping  fields  of  vegetation.  Since  the  settlement  of  the  regions  where 

FIG.   137. 


FIG.   137.     Katydid    (Cyrtophyllus  perspicillatus'),   natural  size.     Photo  by   Folsom. 

Questions  on  the  figure. — How  many  pairs  of  appendages  are  visible 
in  the  figure?  How  many  pairs  are  present?  To  what  order  of  insects 
does  the  Katydid  belong?  What  are  its  feeding  habits?  What  can  you 
find  of  its  development? 

FIG.   138. 


FIG.    138.     Periodical    Cicada.     Natural    size.     Photo    by    Folsom. 

Questions  on  the  figure. — To  which  order  of  insects  does  Cicada 
belong?  Which  of  its  habits  are  most  familiar  to  you?  What  are  its 
nearest  relatives  among  the  insects  ? 

they  breed,  with  the  ploughing  up  of  the  eggs  and  the  destruction  of  the 
young,  there  is  reason  to  hope  that  these  migrations  are  at  an  end.  Ac- 
counts of  similar  migrations  of  locusts  are  recorded  in  the  history  of  the 
old  world.  These  migrations  and  their  effects  illustrate  how  climatic 


296  ZOOLOGY. 

conditions  in  one  locality  may  change  the  balance  of  life  in  another.  The 
second  chapter  of  the  prophet  Joel  gives  a  vivid  account .  of  a  visitation 
of  locusts.  See  Fig.  137. 

Order  Neuroptera  (nerve-wings). — The  members  of  this  order  have 
a  more  or  less  complete  metamorphosis.  Two  pairs  of  netted  membranous 
wings.  The  mouth  parts  are  suited  to  biting.  Here  we  may  include  the 
social  termites  or  white  ants,  the  may-flies,  whose  adult  life  usually  lasts 
onl  /  for  few  hours,  and  the  carnivorous  dragon-flies. 

FIG.  139. 


FIG.   139.     Larvae  of  the  Bot-fly   (Gastrophilus  equi)  in  the  stomach  of  the  horse.     One 
half  natural  size.     From  Luggar,  after  Heller. 

Questions  on  the  figure. — What  do  you  know  of  the  habits  of  the  bot- 
fly? Where  are  the  eggs  deposited?  How  do  the  larvae  come  to  have 
the  position  figured  above?  How  do  they  pass  from  this  to  the  adult 
condition?  See  also  Fig.  140.  How  does  it  retain  its  position  in  the 
stomach  of  its  host? 

Order  Hemiptera  (half-wing}. — Hexapoda  with  an  incomplete  meta- 
morphosis, and  having  two  pairs  of  wings,  or  none.  Mouth  parts  are 
modified  for  piercing  and  sucking.  Here  are  included  the  true  bugs,  as 
the  squash  bug,  the  water  boatman,  etc. ;  the  lice ;  the  plant-lice ;  and  the 
cicadas  (sometimes  called  "locusts").  These  should  not  be  confused  with 
the  beetles,  which  are  often  called  "  bugs."  See  Fig.  138. 

Order  Diptera  (two  wings}. — These  Hexapoda  undergo  a  complete 
metamorphosis,  having  the  anterior  pair  of  wings  developed  (not  in  fleas). 


ARTHROPODA. 


297 


The  posterior  pair  is  very  much  reduced  or  wanting.  The  mouth  parts 
are  well  adapted  for  piercing  and  sucking.  The  order  is  very  large  in 
species  and  includes  such  common  forms  as  the  flies,  mosquitoes,  gnats, 
fleas.  Many  members  of  this  group  are  of  great  importance  to  man.  The 
maggots  of  the  true  flies  are  scavengers,  developing  in  decaying  organic 
matter  and  assisting  in  its  destruction;  the  adults,  on  the  other  hand, 

FIG.   140. 


.  lrrc.  140.  Stages  in  the  development  of  the  Bot-fly  (Gastrophilus  equi).  From 
Parker  and  Haswell,  after  Brehm.  a,  adult  insect;  b,  egg  attached  to  a  hair;  c.d.  and  e, 
stages  in  the  development  of  the  larva.  (See  also  Fig.  139.) 


besides  being  unpleasant  companions  and  demanding  a  share  of  our  com- 
forts, probably  spread  disease.  Other  species  suck  the  blood  of  man  and 
domestic  animals,  producing  disease  and  death.  The  bot-flies  are  most 
important  in  their  larval  stage.  The  eggs,  deposited  on  the  exterior,  are 
taken  into  the  digestive  tract  and  there  develop,  often  migrating  into 
other  organs  and  producing  definite  diseases.  Mosquito  larvae  devour  the 
decaying  organic  matter  in  stagnant  pools.  The  adult,  especially  the  female, 
is  a  blood-sucker  and  is,  through  the  parasitic  protozoa  which  may  infest 
it,  the  chief  instrument  of  the  spread  of  malaria  and  yellow  fever  among 
men.  They  are  all  very  prolific  and  develop  rapidly  considering  the  fact 


298 


ZOOLOGY. 


that  they  undergo  a  metamorphosis.  The  fly,  for  example  only  requires 
a  few  hours  for  hatching  into  the  maggot  stage.  If  food  and  temperature 
are  favorable,  this  maggot  may  grow  to  full  size  in  a  week,  when  it  passes 
into  the  resting  or  pupa  stage,  from  which  another  week  or  more  is  re- 
quired for  the  young  fly  to  emerge. 


FIG.  141.  Two  stages  in  the  metamorphosis  of  the  Mosquito.  From  Packard.  A, 
larva;  B,  pupa;  C,  ventral  view  of  the  oar-like  appendages  of  the  last  segment  of  the 
pupa;  r,  respiratory  tube  of  the  larva;  r',  respiratory  tubes  of  the  pupa. 

FIG.  142. 


FIG.   142.     The    Hessian    Fly    (Cecidiomya   destructor).     From    Standard    Natural    His- 
tory,    a,  the  adult;  b,  larva;  c,  pupa;   d,  larvae  in  position  on  stalk  of  wheat. 

Questions  on  the  figure. — Give  names  to  all  the  structures  apparent 
on  the  adult.  In  which  stage  does  the  insect  do  its  damage?  What  is 
its  economic  importance?  What  is  the  origin  of  its  common  name? 

The  eggs  of  mosquitoes  are  deposited  in  water,  where  they  hatch  into 
active  larvae  called  "  wigglers."  These  breathe  the  air  by  means  of  a  tube 
on  the  next  to  the  last  abdominal  segment.  Their  common  position  with 
the  end  of  the  tail  at  the  surface  of  the  water  is  thus  explained.  The 


ARTHROPODA. 


299 


mosquito  larva  does  not  cease  to  be  active,  but  by  a  series  of  moults  comes 
to  the  so-called  pupa  stage  from  which  by  an  early  moulting  the  adult 
mosquito  emerges,  balancing  itself  on  the  floating  pupal  skin  until  its  wings 
are  hardened  sufficiently  for  use.  See  Fig.  141. 

FIG.   143. 


c 


FIG.   143.     The  Cabbage  Worm  (Pieris  rapa).     Natural  size.     Photo  by  Folsom.     A  and 

B,  larvae;  C,  pupa. 

Questions  on  the  figure.— What  is  a  larva?  What  is  a  pupa?  Which 
is  the  earlier  stage?  What  is  the  color  of  this  caterpillar  in  nature? 
See  the  next  figure  for  the  adult. 

FIG.  144. 


FIG.    144.     The  adult  Cabbage  Butterfly  (Pieris  rapcr).     Natural  size.     Photo  by  Folsom. 

Questions  on  the  figure. — Why  is  the  larva  of  this  animal  called  the 
cabbage  worm?  Why  is  the  adult  called  the  cabbage  butterfly?  What 
are  its  feeding  habits? 

The  Hessian-fly  deposits  its  eggs  in  the  tissues  of  growing  wheat  and 
corn ;  the  clover-gnat  and  others  produce  galls  which  interfere  with  the 
growth  of  the  plant,  often  very  seriously.  In  the  case  of  the  Hessian-fly 
great  damage  to  the  wheat  crop  often  results.  See  Fig.  142. 


300 


ZOOLOGY. 


The  fleas  are  to  be  looked  upon  as  degenerate.  They  are  external 
parasites  without  wings. 

Order  Lepidoptera  (scale-wings}. — These  are  Hexapoda  which  pass 
through  a  complete  metamorphosis,  possess  in  the  adult  sucking  mouth 
parts,  and  have  two  pairs  of  large  membranous  wings  covered  with  scales. 
The  moths  and  butterflies  are  the  representatives  of  the  order.  The  larva 
are  known  as  caterpillars,  which,  with  a  few  exceptions,  are  vegetable 
feeders.  The  adult  butterfly  differs  from  the  moths  (typically)  in  the  fact 
that  the  former  fly  by  day,  hold  the  wings  erect  when  at  rest  and  have 
antennae  with  a  club  on  the  end.  The  butterflies  share  with  the  birds  the 
preeminence  in  beauty  among  animals.  They  present  many  points  of 
interest  in  their  metamorphosis,  in  their  habits,  their  coloration,  their  dis- 
tribution, and  their  economic  effects. 

The  caterpillars  are  usually  voracious  and  may  strip  their  food  plant  of 
its  leaves  and  buds.  The  majority  of  the  larvae  have  become  highly 
specialized  in  their  food  habits,  becoming  restricted  in  some  instances  to 

FIG.  145. 


FIG.   145.     The  Army  Worm   (Leucania  unipuncta).     After  Riley.     A,  caterpillar; 

B,   adult  moth. 

Questions  on  the  figure. — What  are  the  principal  facts  concerning 
the  habits  and  economic  importance  of  the  army-worm?  Why  is  it  so 
called? 

one  species  or  to  a  few  related  species  (as  illustrated  by  the  tomato  worm, 
which  feeds  on  tomato,  potato,  and  tobacco  leaves ;  or  the  cabbage  worm 
which  eats  the  leaves  of  certain  of  the  cruciferous  plants).  The  distribu- 
tion of  such  species  is  thus  clearly  determined  by  that  of  their  host  plants. 
The  most  injurious  to  vegetation  are  the  "  tent-caterpillars  "  which  occur 
gregariously  and  spin  a  web-lilce  nest;  the  army-worm,  so-called  because 
it  appears  and  moves  from  its  hatching  grounds  in  great  numbers;  the 
cotton-boll  worm;  the  canker-worms  and  fruit-borers.  The  silk-worm 
seems  to  be  the  only  useful  member  of  the  order.  The  clothes-moth  lays 
its  eggs  in  woolens  or  furs,  its  larvae  thus  being  exceptional  in  preferring 
animal  diet. 

The  adults  are  usually  short-lived  and  some  do  not  eat  at  all.  The 
majority  of  them  suck  nectar  from  flowers  and  juices  from  ripe  fruits  and 
other  objects  by  means  of  special  tubular  mouth  parts  which  are  modified 


ARTHROPODA. 


301 


paired  appendages.  They  carry  pollen  from  flower  to  flower,  effecting 
cross-fertilization,  in  some  instances.  The  color  of  the  larvae  and  the 
adults  is  very  varied  and  has  close  relation  to  the  environment  and  habits 
of  the  animals.  We  have  already  noticed  in  the  chapter  on  adaptations 
(Chap.  VIII)  how  the  coloration  may  be  protective.  This  is  the  more 
needed  since  the  group  has  many  enemies,  especially  in  the  larval  stage. 
The  power  of  reproduction  is  great.  Several  broods  per  year  may  be 
produced.  There  are  25,000  known  species  of  Lepidoptera,  6,000  of  which 
occur  in  North  America,  north  of  Mexico.  The  species  are  more  numerous 
and  striking  in  the  tropical  regions  of  South  America. 

Order  Coleoptera  (shield-wings}. — In  this  group  there  is  a  complete 
metamorphosis.  The  mouth  parts  are  suited  to  biting  and  chewing.  The 
front  wings  (elytra}  are  hardened  and  serve  as  covers  for  the  true  mem- 

FIG.  146. 


FIG.   146.     Swallow-tail   Butterfly    (Papilio  machaon), — larva,  pupa,   and   adult. 
From  Nicholson. 

Questions  on  the  figure. — Which  is  the  larva  and  which  the  pupa? 
Which  of  these  is  the  earlier  stage?  What  are  the  chief  characteristics  of 
the  three  stages  in  the  metamorphosis  of  butterflies, — the  larva,  the 
pupa  and  the  imago? 


302 


ZOOLOGY. 


branous  wings  when  the  latter  are  not  in  use.  These  are  the  beetles, — 
falsely  called  "  bugs."  Although  a  well-defined  order  the  beetles  are  very 
various,  as  will  be  seen  from  the  fact  that  there  have  been  described  over 
eleven  thousand  species  for  this  continent  north  of  Mexico.  There  are 
said  to  be  more  than  one  hundred  thousand  known  species  of  beetles. 

The  larvae  are  commonly  known  as  grubs.  The  feeding  habits  are 
almost  as  diversified  as  the  form.  Many  are  scavengers  and  lay  their 
eggs  in  carrion  and  other  decaying  matter ;  others  bore  into  wood  and 


FIG.  147- 


FIG.   147.     Hornet's   nest,    sectioned.     Photograph    from   life   by   Shufeldt. 

bark,  as  the  long-horned  beetles;  some  frequent  grain,  nuts,  fruits;  others 
are  leaf-eaters;  a  few  devour  other  insects.  The  Colorado  potato-beetle, 
the  weevils,  the  museum  pest,  the  locust-borer  or  the  hickory-borer  will 
serve  to  illustrate  some  of  the  more  hurtful  representatives  of  this  immense 
order. 

Some  especially  interesting  forms  are  the  fire-flies,  the  scarabeids,  in- 
cluding the  sacred  dung-beetle  of'  Egypt,   and  the   ladybird-beetle.     The 


ARTHROPODA.  303 

latter  is  useful  to  man  owing  to  the  fact  that  it  preys  on  certain  hurtful 
insects.  In  California  the  cottony-cushion  scale,  which  in  some  way  had 
been  imported  from  Australia,  promised  at  one  time  to  destroy  totally 
the  orange  industry.  The  Australian  ladybug,  which  keeps  it  within  bounds 
in  its  native  home,  was  imported,  and  the  increase  of  the  ladybugs  was 
such  that  the  cushion-scales  were  all  but  destroyed.  This  species  of  ladybug 
feeds  exclusively  on  the  cottony-cushion  scale,  and  therefore  the  destruc- 
tion of  the  latter  led  in  turn  to  the  rapid  decline  of  the  ladybugs  from 
the  loss  of  their  food  supply.  Indeed  it  was  necessary  to  keep  colonies 
of  the  scale  insects  protected  in  order  to  furnish  food  and  to  prevent  the 
entire  destruction  of  the  imported  beetle  by  starvation.  In  Australia  where 
both  are  at  home  the  natural  conditions  and  the  adjustment  of  the  two 
species  are  such  that  this  scale-insect  does  not  become  a  pest.  The  dis- 
covery of  the  biological  relations  of  these  species,  and  the  relief  of  the 
orange  industry  furnish  a  sample  of  the  excellent  work  being  done  by  the 
U.  S.  Department  of  Agriculture  in  connection  with  the  economic  aspects 
of  biology. 

Order  Hymenoptera  (Membrane-wings). — Hexapoda  with  four  mem- 
branous wings;  mouth  appendages  adapted  for  sucking  or  for  biting; 
metamorphosis  complete.  This  is  the  most  -highly  developed  division  of 
Insecta,  and  embraces  such  forms  as  bees,  wasps,  and  ants.  The  most 
important  habits  of  the  group,  which  are  those  growing  out  of  their  social 
life,  have  been  referred  to  in  the  chapter  on  adaptations  (Chapter  VIII). 
The  chief  economic' value  of  the  order  is  in  the  honey  of  the  honey-bee, 
the  fertilization  by  bees  of  certain  plants,  as  clover,  and  the  reduction  of 
more  hurtful  species  of  insects  by  certain  parasitic  members  of  the  order, 
as  the  ichneumon-flies.  Some  of  the  larvae  are  leaf-eating,  as  the  rose- 
slug,  and  others  produce  galls  on  the  oak  and  other  platits  in  depositing 
their  eggs.  These  are  harmful  to  human  interests. 

Class  V.  Arachnida  (Spiders,  Scorpions,  etc.). — Arachnida  are  arthro- 
pods in  which  the  head  and  thorax  are  typically  fused  and  represent  about 
seven  segments  with  six  pairs  of  appendages.  There  are  no  antennae.  The 
abdomen  is  often  segmented  but  usually  without  paired  appendages. 
Respiratory  organs  are  confined  to  the  abdomen,  and  may  be  of  three 
types :  book-gills,  associated  with  appendages  (king-crab)  ;  trachea  similar 
to  those  of  insects;  and  book-lungs  (spiders).  Development  is  usually 
direct. 

Order  I.  Xiphosura  (The  King  Crab). — This  order  contains  only  one 
genus,  Limulus,  a  marine  form  with  book-gills,  and  a  cuticular  test  like 
that  of  the  Crustacea,  with  which  it  was  formerly  classified.  Numerous 
related  forms  flourished  earlier  in  the  world's  history  but  are  now  extinct. 

Order  2.  Scorpionida  (Scorpions'). — Arachnids  with  a  much  elongated1 
and  segmented  abdominal  region  closely  connected  with  the  thorax.  They 
are  air-breathers,  with  four  pairs  of  book-lungs  in  the  abdomen.  The 
posterior  abdominal  segments  form  a  tail  the  last  segment  of  which  bears 
a  sting.  See  Fig.  148. 


3^4 


ZOOLOGY. 


Order  3.  Araneida  (Spiders). — The  Araneida  are  air-breathing  arach- 
nids, with  book-lungs  alone  or  in  connection  with  tracheae.  Poison  glands 
are  common  in  connection  with  the  first  pair  of  (mouth)  appendages.  The 
abdomen  is  unsegmented  and  without  appendages,  unless  the  spinnerets 
represent  reduced  appendages.  On  these  latter,  open  the  ducts  of  the 
numerous  glands  secreting  the  fluid  which  hardens  on  exposure  to  the 
atmosphere  and  makes  the  silk  of  the  web. 

FIG.  148. 


FIG.   148.     Scorpion    (Buthus).     Photo  by  Folsom. 

Questions  on  the  figure. — Compare  the  scorpion  with  figures  of  Crus- 
tacea, insects  and  spiders,  noting  the  chief  differences  and  likenesses.  Of 
what  use  is  the  long,  segmented  abdomen  in  the  scorpion? 

Spiders  may  be  classified  on  the  basis  of  the  type  of  web  which  they 
make.  The  "  orb-weavers  "  construct  webs  of  great  regularity  and  beauty ; 
others,  as  the  cob-web  spider,  make  a  complex  and  irregular  mesh-work 
of  fibres  running  in  all  directions;  others  spin  a  web  similar  to  the  last 
with  the  exception  that  at  one  point  it  is  continued  into  a  tube  into  which 
the  spider  retreats  for  hiding.  The  webs  of  these  spiders  are  for  the 


ARTHRGPODA, 
FIG.  149. 


305 


PIG.   149.     Spiders  (Epeira  marmorea).     After  McCook.     Male  on  left;  female  on  right. 

Natural  size. 

Questions  on  the  figure. — What  differences  do  you  note  with  re- 
spect to  the  sexes?  What  habits  of  the  spiders  are  correlated  with  this 
difference  in  size  in  the  sexes? 


FIG.  150. 


FIG.   150.     Web  of  Epeira' strix,  an  Orb-weaving  Spider.     After  McCook. 

Questions  on  the  figure. — By  reference  to  other  texts  or  by  observa- 
tion determine  if  there  is  any  regular  order  in  which  the  parts  of  the  web 
are  produced.  To  what  is  this  form  of  web  an  adaptation?  Evidences? 
What  other  forms  of  webs  are  constructed  for  similar  purposes? 


ZOOLOGY. 

purpose  of  catching  flies  and  other  insects  on  which  the  animal  feeds.  The 
trap-door  spiders  make  a  tunnel  in  the  ground  which  they  line  with  their 
secretion;  a  door  is  woven  which  is  so  covered  with  materials  like  those 
about  the  nest  that  its  presence  is  effectually  hidden.  A  considerable  num- 
ber of  spiders  do  not  spin  proper  webs,  but  use  their  secretion  merely  in 
forming  cocoons  for  their  eggs,  or  in  binding  together  objects  to  make 
a  home.  This  wonderful  secretion  is  used  by  the  spider  in  many  other 
ways  than  in  the  capture  of  prey  and  the  making  of  a  nest.  By  means  of 
it  some  of  the  spiders  make  a  near  approach  to  flying.  A  spider  may 
bridge  the  space  from  one  object  to  another  either  by  fastening  one  end 
of  the  strand  and  hanging  at  the'  other,  or  by  sitting  still,  he  may  allow 
the  free  end  to  float  out  until  it  becomes  attached.  In  some  cases  at  least 
it  is  known  that,  by  spinning  thus  loose  silk  in  abundance,  the  weight  of 
the  spider  may  be  readily  carried  by  the  action  of  the  wind  upon  his 
silken  sails. 

The  chief  economic  importance  of  spiders  lies  in  their  habit  of  preying 
on  various  insects,  of  which  they  destroy  considerable  numbers. 

The  Arachnida  embraces  a  number  of  other  orders  including  less 
important  or  less  easily  observed  animals,  as  the  mites,  certain  ticks,  har- 
vest-men or  "  daddy-long-legs,"  and  many  parasitic  or  otherwise  degen- 
erate forms. 

328.  Suggestive  Studies,  for  Field  and  Library. 

1.  Dimorphism  and  polymorphism  in  insects. 

2.  Protective  adaptations  in  insects. 

3.  What  senses  seem  most  used  among  the  insects? 

4.  Report  on  observed  signs  of  intelligence  among  arthro- 
pods. 

5.  Is  there  any  evidence  of  power  of  communication  among 
the  social  insects,  as  the  ants? 

6.  Courtship  among  the  spiders. 

7.  Spiders'  webs :  form,  position,  efficiency,  mode  of  con- 
struction. 

8.  There  are  some  insects  which  have  wings  during  a  por- 
tion of  their  life  but  lose  them  later.     Investigate  the  condi- 
tions and  find  an  explanation. 

9.  Report  an  observed  instance  of  insects  fertilizing  flowers 
(i.  e.,  transferring  pollen  from  one  to  another).  How  is  it 
effected?   Why  does  the  insect  do  it?   Is  the  fertilization  of 
flowers  by  insects  deemed  a  common  and  important  phenome- 
non by  botanists? 


ARTHROPODA.  307 

10.  Can  you  find  any  recorded  instances  of  what  may  be 
called  symbiosis  (see  §  156)  between  insects  and  other  organ- 
isms? 

11.  Have  you  any  experimental  evidence  as  to  how  growth 
can  take  place  in  forms  with  a  firm  external  skeleton  such  as 
that  of  the  cray-fish? 

12.  Do  you  have  any  reason  for  thinking  that  a  metamor- 
phosis is  advantageous  to  any  of  the  Arthropoda?  Is  it  in  any 
respect  disadvantageous  ? 

13.  How  is  moulting  effected? 

14.  Habits  of  the  "hermit  crabs." 

15.  The  lobster  and  its  habits. 

1 6.  Silkworm  culture,  value  and  methods  of. 

17.  Insects  introduced  from  foreign  countries. 

1 8.  The  history  of  the  efforts  to  find  enemies  to  some  of 
the  more  important  noxious  insects. 

19.  Relation  of  insects  to  the  culture  of  figs. 

20.  The  structure  and  habits  of  the  king  crab  (Limulus). 
Why  is  it  not  to  be  classed  with  the  true  crabs  ? 


CHAPTER   XVIII. 

PHYLUM  VIII.— CHORDATA. 

329.  This  phylum  includes,  beside  the  typical  Vertebrata 
to  be  described  in  later  chapters  (fishes,  amphibians,  reptiles, 
birds  and  mammals),  several  groups  of  much  more  simple 
organization.     These  latter  forms  may  be  included  under  the 
general  head  Protovertebrata,  not  because  they  all  show  close 
relationship  among  themselves,  but  because  of  their  primitive 
character,  considered  as  chordata.     They  are  of  very  great 
interest  to  the  biologist  on  account  of  the  hints  they  may 
offer  concerning  the  ancestors  of  the  Vertebrates. 

330.  General  Characters  of  the  Chordata  (Protoverte- 
brata and  Vertebrata). — The  Protovertebrata  are  allied  with 
the  typical  vertebrates  and  separated  from  the  invertebrates  by 
the  possession,  either  in  the  larval  or  adult  condition,  of  the 
following  features : 

1.  A  mid-dorsal,  longitudinal  rod  of  cells  (notochord)  de- 
rived from  the  entoderm,  but  often  surrounded  by  mesodermal 
structures  (see  Fig.  155).    This  lies  ventral  to  and  supports,— 

2.  The  central  nervous  system,  a  mid-dorsal  cellular  tube 
with  thickened  walls  derived  from  the  ectoderm. 

3.  Gill-slits    or    perforations    connect    the    cavity    of    the 
pharynx  with  the  outside  directly  or  through  an  atrial  cham- 
ber. 

4.  .The  heart  is  typically  ventral  to  the  digestive  tract. 

331.  In  the  Group  of  Pro  to  vertebrates  may  be  placed: 

i.  Balanoglossus.  a  soft-bodied,  worm-like  form  whose 
claim  to  a  place  among  the  Chordata  rests  upon  the  fact  that 
an  outgrowth  of  the  gut  extends  into  the  proboscis,  where  it 
forms  a  solid  rod  which  in  its  origin  suggests  the  notochord; 
a  portion  of  the  nervous  system  is  dorsal ;  and  gill-slits  occur. 

308 


CHORDATA.  309 

On  the  other  hand  there  is  a  connective  around  the  oesophagus 
and  a  ventral  nervous  cord  as  in  Annulata,  and  it  shows  no 
signs  of  segmentation  (see  Fig.  151). 

2.  Tunic  at  es  (sea-squirts,  ascidians,  etc.)  comprising  a 
variety  of  forms  which  may  be  said,  on  the  whole,  to  be  de- 
generate in  the  adult  condition.  It  is  in  the  larval  or  tadpole 

FIG.  151. 


FIG.   151.     Balanoglossus    (Male).     After    Bateson.     a,   anus;    m,    mouth;    p,   proboscis; 
po.,    pores,   the  openings   of  the   gill-slits;    ts.,   testes. 

Questions  on  the  figure. — Make  reference  to  other  texts  and  figures 
and  determine  what  features  of  Balanoglossus  tend  to  ally  it  with  the 
chordates.  What  are  the  habits  of  the  animal?  Where  do  the  earlier 
zoologists  class  Balanoglossus? 

state,  particularly,  that  their  relation  to  the  chordata  is  sug- 
gested. In  the  larva  they  possess  a  notochord  especially  in  the 
tail  region,  a  dorsal  nervous  system,  and  gill-slits.  The  adult 
forms  are  usually  attached,  many  of  the  larval  organs  be- 
coming much  changed  or  even  wholly  lost  in  consequence  of 
the  changed  mode  of  life.  The  adults  have  been  variously  clas- 
sified as  worms,  mollusks,  etc.  Many  of  the  tunicates  multiply 
by  budding  and  form  colonies  from  the  fact  that  the  buds 
remain  associated. 

3.  Amphioxus  (lancelet)  possesses  the  characters  above 
mentioned  as  belonging  to  all  the  Chordata,  except  that  there 
is  no  true  heart.  In  addition  it  has  a  fish-like  body,  and  the 


3io 


ZOOLOGY. 


FIG.  152. 


L-e 


muscles  are  arranged  in  segments  which 
appear  externally.  It  lacks  paired  fins, 
but  has  a  median  dorsal  fin  which  con- 
tinues over  the  tail  and  forward  a  part 
way  on  the  ventral  side.  In  front  of  this 
is  a  thick  fold  (inetaplenre)  on  either  side 
the  body,  at  the  junction  of  the  side  with 
the  belly  of  the  animal.  The  metapleure 
is  thought  by  some  zoologists  to  be  the 
forerunner  of  the  vertebrate  appendages. 
Amphioxus  is  without  a  definite  brain, 
that  is  to  say,  the  anterior  end  of  the  ner- 
vous tube  is  not  highly  specialized.  It 
has  no  skull,  eyes,  nor  ears,  such  as  char- 
acterize the  head  of  true  vertebrates. 
Amphioxus  is  a  small  semi-transparent 
animal  about  two  inches  long  (Fig. 
152).  It  burrows  in  the  sand  with  only 
the  fringed  mouth  exposed.  It  may  vary 
this  by  swimming  about  for  short  periods. 

FIG.  152.  Diagram  of  the  anatomy  of  Amphioxus, 
drawn  as  a  semi-transparent  object  (after  Perrier  "  Traite 
de  Zoologie").  a,  anus;  a.p.,  atrial  pore;  c.f.,  caudal  fin; 
dr.,  cirri,  on  the  edge  of  the  vestibule  leading  to  the 
mouth;  d.f.,  dorsal  fin;  r/^fin  rays;  g,  gill  or  branchial 
structures  consisting  of  alternate  slits,  through  which  the 
water  passes,  and  supporting  plates,  in  the  walls  of  which 
are  the  blood  vessels;  in.,  'intestine,  from  which  as  a 
diverticulum  springs/.,  the  liver;  m,  the  mouth  surrounded 
by  a  fringed  velum;  my.,  myotomes  or  muscle  segments; 
n.c.,  notochord;  o.,  ovaries;  s.c.,  spinal  cord;  v.f.,  ven- 
tral hn. 

Questions  on  the  figure. — What  elements  of 
structure  appear  in  the  -figure  suggesting  the 
chordate  character  of  Amphioxus?  What  is  the 
relation  of  the  spinal  cord,  notochord  and  diges- 
tive tract?  How  much  of  the  length  of  Amphi- 
oxus possesses  gills?  What  is  the  position,  ex- 
tent and  function  of  the  atrium?  (Refer  to  more 
extended  texts.)  What  structures  show  evi- 
dences of  segmentation?  What  fins  has  Amphi- 
oxus? Compare  with  fins  of  fishes. 


CHORDATA.  311 

4.  Cyclostomes  (Lampreys). — These  are  eeKlike  animals 
usually  classed  with  the  fishes,  and  are  doubtless  more  closely 
related  to  them  than  to  the  forms  before  mentioned,  but  be- 
cause of  their  primitive  qualities  they  may  be  placed  for  the 
purposes  of  this  course  among  the  Protovertebrata.  They 
have  a  round  sucking  mouth  destitute  of  jaws;  they  lack 
paired  appendages  and  the  external  skeleton.  There  is  only 
one  nostril,  which  may  or  may  not  communicate  with  the 
pharynx.  The  cyclostomes  possess  a  true  brain,  a  cartilagi- 
nous, internal  skeleton,  and  gills  (usually  6  or  7  pairs)  in 
pouches.  They  differ  from  the  true  fishes  in  the  fact  that  the 
notochord  is  not  constricted,  i.  e.,  the  mesodermal  sheath  does 
not,  by  its  growth,  compress  it  by  the  development  of  distinct 
vertebrae  around  it  (see  336).  See  figure  62. 

332.  Library  Exercises. — By  reference  to  all  the  available 
literature  make  a  report  on  the  general  structure,  habits,  and 
important  adaptations  of  each  of  the  above  types?  How  do 
the  larvae  and  adults  of  the  tunicates  compare?  How  is  the 
degeneration  accounted  for?  To  what  extent  is  colonial  life 
represented  among  these  types  ?  Are  any  parasitic  ?  Examine 
particularly  for  figures  of  these  groups  in  the  standard  refer- 
ence zoologies. 


CHAPTER   XIX. 

CHORDATA   (CONT.)  :  SUB-PHYLUM  VERTEBRATA   (FISHES,  AM- 
PHIBIANS, REPTILES,  BIRDS,  AND  MAMMALS). 

LABORATORY  EXERCISES. 

For  general  illustration  of  the  vertebrates  the  author  is 
convinced  that  no  form  is  superior  to  the  frog  for  use  in 
elementary  classes,  although  some  teachers  prefer  a  fish.  In 
a  course  arranged  for  one  year  it  is  not  desirable  to  make 
elaborate  dissections  of  more  than  one  vertebrate  type.  Direc- 
tions are  given  both  for  the  fish  and  the  frog  for  the  con- 
venience of  those  teachers  who  prefer  the  former.  Supple- 
mentary studies  for  the  other  classes  of  vertebrates  will  be 
found  in  connection  with  the  chapters  devoted  thereto. 

333.  Fish. — Any  common  fish  will  serve — perch,  sucker,  trout,  smelt. 
Specimens  eight  to  ten  inches  in  length  are  of  most  suitable  size.  Tf 
convenient  one  half  the  class  might  take  one  species  and  the  remainder 
another. 

A.  The  Living  Animal. — Place  in  a  tub  of  water,  or  better  in  a  vessel 
one  side  of  which  is  glass.     Watch  the  locomotion  and  notice  all  the  ac- 
companying motions  of  the  various  parts.     What  is  the  rate  of  the  tail 
stroke?    How  far,  on  an  average,  does  one  stroke  of  the  tail  carry  the 
fish?    Compare  these  points  when  the  fish  is  in  very  rapid  motion.     What 
part  do  the  anterior  fins  play  in  locomotion?   Bind  one  of  them  flat  against 
the  body  with  a  string.     Bind  both.     Results?    Experiment  similarly  with 
the  other  fins  and  see  if  your  first  conclusions  are  strengthened.     Do  you 
find  any  variations  in  the  above  respects  by  comparing  several  species? 

How  does  the  temperature  of  the  fish  compare  with  that  of  the  water? 
t  Allow  one  specimen  to  remain  for  an  hour  or  more  in  water  at  a  tem- 
perature of  70°  F. ;  another  in  cooler  water  (50°  F.)  :  compare  results. 

Can  the  fish  detect  in  the  water  the  presence  of  substances  which  have 
a  decided  taste  to  us?  Use  colorless  solutions, — acid,  sugar,  quinine.  Can 
you  get  the  animal  to  show  any  choice  as  to  food? 

Note  the  motions  of  mouth  and  eyes.  Can  the  fish  see  any  point  with 
both  eyes  at  once? 

B.  External   Anatomy. — (Make    careful    outline    sketches    showing    all 
points  of  structure.) 


CHORD  AT  A.  313 

The  Topography  of  the  Body. — Note  the  symmetry;  indicate  the  degree 
of  differentiation  of  anterior  and  posterior  ends,  and  of  dorsal  and  ventral 
surfaces,  as  shown  by  the  shape,  special  organs,  etc.  What  structures 
appear  paired?  To  what  degree  are  head,  trunk,  and  tail  clearly  distin- 
guishable? Locate  and  identify  all  the  external  openings.  How  would 
you  describe  the  general  shape  of  the  body?  What  are  the  external  evi- 
dences of  segmentation? 

The  Appendages. — How  many  pa'ired?  Unpaired?  Locate:  the  dorsal, 
caudal,  anal,  the  pectoral,  and  the  pelvic  or  ventral.  Does  the  skin  of  the 
body  extend  over  the  fins?  What  seems  to  be  the  nature  of  the  fin  rays? 
Number?  Are  the  upper  and  lower  lobes  of  the  caudal  fin  equal  or 
unequal  ? 

The  Covering. — Does  the  specimen  possess  scales?  Is  there  any  regu- 
larity in  their  arrangement?  Is  this  constant  among  several  specimens? 
Is  it  the  same  in  different  species?  Are  any  parts  of  the  body  free  of 
scales?  Are  the  scales  covered  with  skin?  What  is  the  shape  and  nature 
of  the  free  margin  of  the  scales?  Examine  with  a  hand  lens  or  low  power 
of  microscope.  Is  there  any  color?  How  does  this  appear  under  the 
microscope?  Do  you  distinguish  a  line  (lateral  line}  along  one  of  the 
rows  of  scales  on  the  side  of  the  body?  Examine  one  of  these  scales  under 
the  microscope.  How  does  it  differ  from  the  others?  How  many  rows  of 
scales  above  the  lateral  line?  Below? 

The  Head. — What  goes  to  make  up  the  head  of  the  fish?  Note  posi- 
tion, shape  and  size  of  mouth.  Find  the  bony  frame-work:  upper  jaw 
(premaxillaries  in  front,  articulating  with  maxillaries  behind)  ;  lower  jaw 
(dentaries} .  Are  both  jaws  movable?  Locate  all  the  bones  which  bear 
teeth?  How  are  teeth  arranged?  Is  there  a  tongue?  Do  the  nostrils 
communicate  with  the  mouth  cavity? 

Eyes:   number,  position,  coverings    (lids?),   iris,  pupil. 
Are  there  any  "  ears  "  ?    Evidences  ? 

Gills  and  Gill- coverings. — How  many  bones  in  the  gill  cover  (opercu- 
lum)  ?  Describe  the  structure  at  its  inner,  lower  margin.  How  wide  is 
the  isthmus  between  the  right  and  left  gill-openings?  Identify  and  num- 
ber the  gill  arches,  which  bear  the  reddish  gill-filaments,  and  the  gill  slits 
between.  With  what  do  the  passages  between  the  gill  arches  communicate? 
Determine  by  extending  a  probe  into  mouth.  How  does  the  inner  or 
pharyngeal  side  of  the  gill  arches  differ  from  that  which  bears  the 
filaments?  Examine  the  gills  for  parasites. 

C.  Internal  Structure. — With  forceps  and  sharp  scalpel  remove  a  strip 
of  skin  an  inch  wide  from  one  side  of  the  fish,  from  the  belly  to  the  dorsal 
fin.  Note  the  muscular  segments  (myotomes).  Is  the  line  separating 
two  of  these  a  straight  line  ?  In  what  direction  do  the  muscle  fibres  run  ? 

With  scissors  cut  the  body  wall  along  the  middle  line  of  the  belly  from 
just  in  front  of  the  anus  to  the  isthmus.  Be  very  careful  not  to  injure 
any  of  the  organs  within  the  coelom.  A  portion  of  the  side  muscles  may 
be  removed  on  one  side  or  cuts  may  be  made  perpendicular  to  the  first  so 


314  ZOOLOGY. 

that  the  sides  may  be  more  readily  opened.  Notice  the  lining  of  the  body 
cavity  (peritoneum'}.  Color?  How  is  the  heart  separated  from  the  ab- 
dominal cavity?  (False  diaphragm.}  Sketch  the  cavities  thus  laid  open 
and  represent  the  organs  as  they  appear  before  disturbing  them.  Has  the 
liver  lobes? 

Examine  more  in  detail,  turning  liver  to  one  side. 

1.  Digestive  Organs. — Extend  probe  through  the  mouth  into  stomach, 
and  locate:    oesophagus;   stomach,  form  and  position;   intestine,  its  point 
of  origin,  course  and  outlet.     About  the  junction  of  stomach   and  small 
intestine    look    for    finger-like    projections    from    the    surface    of   the    gut 
(pyloric  area).     If  present,  cut  one:  is  it  solid  or  hollow?    Examine  the 
membrane  (mesentery)  which  holds  the  intestine  in  place.     To  what  part 
of  the  body  wall  is  it  attached?    The  spleen,  a  dark-red,  ductless  gland, 
occurs  close  to  the  intestine.     Cut  intestine  an  inch  from  the  anus  and  the 
oesophagus   in   front   of   stomach.     Remove,    open,    and    examine   interior. 
Figure  differences  in  different  portions.    Look  for  parasites  in  the  tract. 

2.  Reproductive   and   Excretory    Organs. — Find    the    whitish    testes    or 
the  yellow  or  pinkish  ovary  (or  ovaries).     Do  they  possess  ducts?    Where 
is  the  outlet? 

Observe  position  and  dimensions  of  the  air-bladder  (if  present)  :  pierce 
it  and  discover  dorsal  to  it  the  red  kidneys.  Number,  shape,  and  dimen- 
sions of  these?  Can  you  find  their  outlets? 

3.  Pericardia!  Cavity  and 'its  Contents. 
Shape  and  boundaries  of  the  cavity. 

Heart:    position;    portions;    ventricle    (ventral)    and    auricle    (more 

dorsal).     From  the  ventricle  the  whitish  bulbus  arteriosus  passes 

forward  and  narrows  into  the  ventral  aorta.     Back  of  the  heart  is 

the  thin-walled  sinus  venosus  which  communicates  with  the  auricle. 

[The  teacher  should  supplement  this  work  by  a  demonstration  of  the 

ventral  aorta  with  its  branches  passing  to  the  gills,  by  means  of  a  larger 

fish  in  which  these  vessels  have  been  injected  with  a  colored  mass.     See 

appendix.] 

4.  The  Nervous  System.— Cut  off  the  head  and  remove  the  muscles  from 
the  back  and  top  of  the  skull.     Use  a  strong  cartilage  knife  and  gradually 
slice  and  pick  the  bone  until  the  cavity  within  is  well  uncovered.     Note 
the  loose  tissues  covering  the  brain.     Remove  this  with  great  care. 

Beginning  in  front,  identify  as  you  pass  backward :  olfactory  lobes, 
tapering  toward  the  front  and  communicating  with  the  nasal  cavities; 
cerebrum,  two  oval  prominences  meeting  in  the  middle  line;  the  two 
large,  rounded  optic  lobes;  the  cerebellum,  a  single  median  lobe;  and  the 
medulla  oblongata,  which  tapers  backward  into  the  spinal  cord.  Is  there 
any  real  boundary  between  the  spinal  cord  and  the  medulla,  or  is  the  dis- 
tinction arbitrary?  What  is  the  size  of  the  cord  where  it  emerges  from 
the  cranium?  What  is  its  position  in  relation  to  the  vertebrae?  Have  you 
found  any  nerves  leaving  the  medulla  or  the  cord?  If  so  how  many? 
Are  there  any  cavities  in  the  brain  lobes? 


CHORDATA.  315 

5.  The  Eye. — Remove  the  bone  from  above  the  eye  and  examine  it  in 
position.     How  is  the  eye  moved  in  life?    Can  you  discover  any  of  the 
muscles   effecting  these  motions?    How  are  they  attached?    What  is  the 
shape  of  the  eye?    Split  it  open,  and  find  the  lens.     Is  the  lens  more  or 
less  nearly  spherical  than  you  expected? 

6.  The  Skeleton. — The  general   shape   and   character  of  the   skull   and 
its  bones  may  be  seen  by  boiling  the  head  of  another  fish  for  a  few  minutes 
and  scraping  and  picking  away  the  flesh.     The  principal  regions  are  the 
cranium  or  brain   case,   the   opercular  bones   of  the  gill   covers,   and   the 
facial  bones.     Notice  the  loose  way  in  which  the  lower  jaw  is  articulated. 

Boil  a  two  inch  block  taken  from  the  tail  of  the  fish  for  several 
minutes.  Notice  incidentally  the  shape  of  the  individual  myotomes  or 
muscle  segments  as  they  fall  apart.  Clean  the  vertebrae  of  flesh,  and  study 
the  structure  of  one  of  them.  Note  the  centrum;  the  dorsal  or  neural 
arch  and  spine ;  the  ventral  or  hamal  arch  and  spine.  What  is  the  shape 
of  the  centrum?  What  structures  occupy  the  arches?  Prepare  a  trunk 
vertebra  and  compare  in  all  respects  with  the  caudal.  How  are  the  ribs 
related  to  the  vertebra  ?  Can  you  find  any  evidence  whether  they  are 
homologous  with  the  haemal  processes  ? 

Are  there  any  bones  connected  with  the  fins? 

D.  General  Questions. — What  internal  organs  show  segmentation?  Do 
they  show  it  equally  in  all  parts  of  the  body?  Do  the  internal  organs 
show  bilateral  symmetry  as  completely  as  the  external?  How  do  you 
account  for  the  fact?  Compare  the  relative  position  of  the  anterior  and 
posterior  appendages  in  as  many  species  of  fish  as  you  can  secure?  What 
are  the  habits  of  the  species  you  have  been  studying?  Feeding  habits; 
spawning  and  breeding  habits  ?  What  are  its  nearest  relatives  among  .the 
fishes? 

334.  The  Frog  (Rana). — Any  species  of  frog  will  serve. 
For  internal  anatomy  as  large  specimens  as  possible  should  be 
used.  The  frog  is  especially  suitable  to  represent  the  verte- 
brates because  of  its  metamorphosis  from  a  water-breathing 
or  fish  habit  into  the  air-breathing  condition,  and  the  readiness 
with  which  the  main  facts  of  this  metamorphosis  may  be  fol- 
lowed even  by  an  elementary  class.  Frogs  may  be  kept  alive 
almost  indefinitely,  even  through  the  winter,  by  putting  them 
in  a  deep  box  covered  with  netting,  in  which  a  pan  of  water 
is  placed.  The  bottom  of  the  box  should  be  covered  by  sod 
or  moss  which  must  be  kept  moist.  Change  the  water  in  the 
pan  every  few  days.  Do  not  place  large  and  small  frogs  in 
the  same  box,  as  the  small  ones  are  more  than  likely  to  dis- 
appear. Unless  living  animals,  as  grasshoppers  and  the  like, 


ZOOLOGY. 

can  be  given  them  it  is  scarcely  worth  while  to  try  to  feed 
them.  They  seem  to  do  quite  as  well  without  food  for  a 
reasonable  length  of  time. 

A.  The  Living  Animal  (chiefly  physiology). — Record  what 
you  know  from  observation  of  the  animal's  general  haunts 
and  habits.  To  what  extent  is  it  a  terrestrial  animal? 
Aquatic?  What  is  the  natural  position  when  at  rest?  What 
are  its  modes  of  locomotion  on  land?  Place  on  the  floor,  and 
test.  Describe  its  motions  in  water,  and  the  use  made  of  the 
parts  of  the  body  in  swimming  and  in  its  other  methods  of 
locomotion.  Can  it  rest  at  the  surface  of  the  water?  How 
much  of  the  body  protrudes  from  the  water?  How  does  it 
dive?  Can  you  find  any  evidence  that  it  does  any  thing  to 
increase  its  specific  gravity  when  diving? 

Feed  by  placing  living  grasshoppers  or  flies  in  the  vessel 
with  a  frog,  or  by  dangling  a  piece  of  meat  in  front  of  it  at 
the  end  of  a  string.  Note  the  action  of  the  tongue  in  making 
the  capture.  Examine  the  mode  of  attachment  of  tongue, 
and  suggest  its  possible  advantages. 

Watch  the  animal  while  floating  at  the  surface  of  water  or 
out  of  water.  Can  you  detect  any  signs  of  breathing?  Note 
carefully  the  nostrils,  the  cheeks  and  the  sides  of  the  ab- 
domen, and  determine  how  it  gets  air  into  its  lungs.  De- 
termine what  senses  are  represented  in  the  frog.  How 
does  it  react  to  salines,  acids,  sweets,  bitters?  Judging  from 
the  position  of  the  eyes  and  from  experiment,  can  a  frog  on 
the  ground  see  objects  in  all  directions?  Can  it  do  so  while 
floating  on  the  surface  of  the  water?  Are  the  eyes  movable? 
Can  the  frog  see  any  point  with  both  eyes  at  the  same  time? 
Select  a  small  frog  and  chloroform  it  until  quiet,  but  do  not 
kill  it.  Wrap  it  in  a  wet  cloth,  and  place  on  a  support  of  such 
height  as  will  allow  the  web  to  be  stretched  over  the  opening 
in  the  stage  of  the  microscope.  With  the  low  power  note  the 
pigment  cells  and  blood  vessels.  Determine  which  are  arteries 
and  which  veins;  present  your  evidences.  By  placing  a  little 
water  and  a  cover-glass  on  the  web  the  high  power  may  be 


CHORDATA.  317 

used,  and  the  behavior  of  the  corpuscles  studied  as  they  pass 
through  the  capillaries.  Similar  studies  may  be  made  on  the 
gills  of  very  young  tadpoles. 

B.  External  Anatomy. — What  is  its  symmetry?    Compare 
carefully  the  structure  and  form  of  the  dorsal  with  that  of 
ventral  surface;  similarly  those  of  the  anterior  and  posterior 
ends.     Compare  several  individuals  as  to  shape,  color  mark- 
ings, size,  etc. 

General  form. 

Head,  trunk,  limbs.     Is  there  any  neck? 

Anterior  appendages:  arm,  forearm,  hand  (including 
digits).  Compare  with  your  own  hand,  and  deter- 
mine which  is  the  first  digit,  or  the  thumb  side  of 
the  hand. 

Posterior  appendages :  thigh,  shank,  ankle,  foot.     How 
many  digits?   Which  is  the  first?   How  many  joints 
in  each?    What  other  peculiarities  are  noteworthy? 
Special  head  structure. 

Mouth ;    position,    dimensions,    degree    of    extensibility ; 

tongue;  teeth,  where  located? 

Sense  organs :  position  of  eyes,  ears,  nose.     Do  the  nasal 
openings  communicate  with  the  mouth?     Pierce  the 
tympanic  membrane  and  discover  with  what  the  open- 
ing communicates. 
Cloacal  opening. 

C.  Internal  Anatomy. — Make  a  slit  in  the  skin  of  the  ven- 
tral surface  from  a  point  just  in  front  of  the  cloaca  forward 
to  the  throat,  a  little  to  one  side  of  the  middle  line.     Make 
incisions  perpendicular  to  this  and  turn  the  flaps  back  to  show 
the  muscles  beneath.     Is  the  skin  as  closely  attached  to  the 
muscles  as  in  the  fish?   Do  you  find  myotomes  as  in  the  fish? 
Draw  in  outline  some  of  the  more  important  muscles  of  the 
chest  and  abdomen.     Cut  the  muscular-  wall  in  the  same  way, 
passing  to  one  side  of  the  breast  bone.     Turn  back  the  flaps 
and  sketch  and  identify  the  organs  in  their  position  in  the 
coelom. 


ZOOLOGY. 

1.  Digestive    Organs. — Pressing   the   liver   aside,    identify 
the  following  parts  of  the  digestive  tube :  oesophagus,  stomach, 
small  intestine,  large  intestine.     Are  there  any  sharp  bound- 
aries between  these  parts? 

Compare  the  lengths  of  the  different  portions.  Find  the 
mesentery,  and  show  its  relation  to  the  intestines.  What  is 
the  relation  of  the  liver  to  the  digestive  tube?  Find  the  gall- 
bladder :  does  it  have  any  duct  leading  to  the  tube  ?  What  is 
the  position  of  the  light  colored  pancreas?  Of  the  darker 
spleen  (this  organ  has  no  duct)  ? 

Cut  the  large  intestine  about  an  inch  from  the  anal  open- 
ing and  the  oesophagus  in  front  of  the  stomach;  remove  the 
tract  from  the  body.  Split  it  from  end  to  end,  wash  it  of  its 
contents  and  describe  and  make  drawings  of  the  interior.  Do 
you  find  any  internal  parasites  ? 

2.  Urino genital    Organs, — Without    removing    any    other 
organs  identify: 

The  kidneys :  color,  form,  position.     Do  they  have  any 

outlet?  The  bladder;  position  and  general  structure. 
The  fat  bodies:  position?    With  what  connected? 
In  the  male : 

Testes;  yellowish,  rounded  bodies.     With  what  organ 

connected  ? 
How  in  a  fresh  specimen  can  you  be  sure  you  have 

found  the  testes  ? 
In  the  female : 

Ovaries,  which  vary  much  in  size  and  appearance  with 
the  time  of  year.  What  are  their  position  and  at- 
tachments ? 

Oviducts.      Do    these    communicate    with    the    body- 
cavity  ?  How  do  they  communicate  with  the  exterior  ? 
Is  there  any  trace  of  the  oviducts  in  the  male? 
3.   The  Lungs. — Open  the  mouth,  find  the  glottis,  insert  a 
blow -pipe,  and  inflate  the  lungs :  number,  position  and  shape  ? 
Cut  them  open  and  examine  the  inner  surface. 
4.   The  Circulatory  System. 


CHORDATA.  319 

The  heart:  Does  this  organ  lie  free  in  the  body  cavity? 
What  is  the  shape  of  the  heart?  To  what  is  it  attached? 
Identify  the  auricles  in  front,  and  the  ventricle  behind.  Can 
you  recognize  the  aorta  arising  from  the  ventricle;  and  the 
venous  sinus  dorsal  to  the  heart  and  receiving  the  large  veins? 
How  many  chambers  to  the  heart  ?  Their  relation  to  each 
other  ? 

Further  study  can  be  pursued  successfully  only  by  injecting 
the  vessels  with  a  colored  mass.  A  specimen  thus  injected 
and  dissected  by  the  teacher  should  be  used  to  demonstrate 
the  three  aortic  arches  (carotid,  systemic  and  pulmonary),  the 
dorsal  aorta  and  its  chief  branches. 

5.  Muscle. — Strip  the  skin   from  the  leg  like  a  stocking. 
Without  cutting,  separate  the  muscles  from  each  other,  demon- 
strating their  general  shape  and  the  tendons  at  the  ends  by 
which  they  are  connected  with  the  bones.     The  end  attached 
to  the  least  movable  bone  is  the  origin,  the  other,  the  insertion. 
What  is  the  origin  and  what  the  insertion  of  the  large  muscles 
of  the  thigh?    Are  the  muscle  fibres  plain  or  cross-striate ? 
(Examine  a  small  bit  under  the  microscope  after  teasing  it 
apart  as  much  as  possible.) 

6.  Nervous  System. — Remove    with   great   care   the   skin,   muscles   and 
bone  from  the   roof  of  the  skull   so  as  to  expose  the  brain.     Continue 
backward  and  expose  the  anterior  portion  of  the  spinal  cord.     Sketch,  as 
it  appears  from  above,  and  identify,  beginning  with  the  anterior  end : 

Olfactory  lobes. 

Cerebral  hemispheres;  number,  size,  form.     Are  they  separate? 

Optic  lobes. 

Cerebellum ;  a  narrow  transverse  band. 

Medulla  oblongata,  tapering  into  the  spinal  cord. 

Examine  the  nerves  arising  from  the  spinal  cord.  What  is  their  posi- 
tion in  relation  to  the  vertebrae?  How  many  pairs  can  yon  discover? 
Does  each  arise  by  a  single  or  double  root?  Find  the  large  nerve  (sciatic) 
which  is  the  chief  nerve  of  the  hind  leg.  How  many  spinal  nerves  enter 
into  the  formation  of  it?  Seek  a  similar  plexus  in  connection  with  the 
front  leg. 

Dissect  the  bone  and  muscle  from  one  side  of  the  skull,  showing  the 
cranial  nerves.  Begin  at  the  anterior  end  and  identify:  (i)  the  olfactory 
nerve;  cut,  and  lift  the  brain  slightly,  showing  (2)  optic  nerves.  Cut 
these  as  far  from  the  brain  as  possible. 


320  ZOOLOGY. 

Note  other  smaller  nerves  and  cut  these.  How  many  are  there?  From 
what  part  of  the  brain  do  the  majority  of  them  arise?  Do  the  optic 
nerves  join  at  the  point  where  they  enter  the  brain? 

7.  Skeleton.— Pick   the   bulk   of   the    flesh    from   the   bones    of   an    un- 
injured   skeleton.      A    few   minutes    of   boiling   or   two    or    three   days   of 
soaking   in    water    will    be    of   advantage    in    the    final    stages    of   cleaning. 
Identify  the  axial  skeleton  and  the  appcndicnlar.     Do  the  appendages  unite 
directly  with  the  axial   skeleton?     Count  the   vertebrae.     To   what   extent 
do  they  differ?     Can  they  be  grouped  into  regions?     Select  a  typical  one, 
and  draw  from  various  positions  to  show  structure.     Do  any  bear  ribs? 
Describe  the  posterior  bone  in  the  series.     Identify  the  parts  of  the  an- 
terior and  posterior  limbs  and  girdles  by  referring  to  Fig.  159,  and  see  to 
what    extent    they   depart    from   the   type    described    there.     Make    outline 
sketches  of  all  the  bones  of  the  right  girdles  and  appendages.     What  is 
the  nature  and  action  of  the  various  joints   of  the  limbs?     In  the   skull 
notice  how  small  a  portion  is  brain-case.     How  is  the  great  width  of  the 
head  secured?     How  is  the  lower  jaw  related  to  the  skull?     Make  a  sketch 
showing   the   proportions    of   these   various   parts.     In    what    position    are 
teeth  borne  ?    Examine  the  sternum  or  breast  bone.     How  related  to  the 
girdle?     Of  what  parts  is  it  composed?     How  much  of  it  is  cartilaginous? 

8.  Development. — Eggs  of  frogs  and  toads  may  be  found  in  the  early 
spring   in  ponds  or  sluggish   streams,   floating  or   attached  to   submerged 
objects.     They  occur  in  slimy  strings  or  masses,  each  egg  enveloped  in  a 
jelly-like   covering.     Transfer  these   to   the   laboratory,   and   keep   covered 
with  water  in  a  shallow  vessel.     Change  the  water  frequently,  and  keep 
a  close  watch  on  the  changes  which  they  undergo.     After  hatching  keep 
water  plants  in  vessels  for  the  tadpoles  to  eat. 

Note  appearance  of  the  egg  (with  low  power  of  microscope). 
Gelatine;  outer  layer,  not  really  a  part  of  the  egg. 
Fertilized  ovum ;  the  darker  interior  sphere,  of  protoplasm  and  yolk. 

If  the  eggs  are  recently  laid,  the  beginning  of  segmentation  will  furnish 
an  interesting  demonstration  for  the  class.  How  are  the  first  cleavage 
planes  related  to  each  other? 

If  more  advanced,  note  especially:  the  gradual  elongation  of  the 
embryo,  the  enlargement  of  the  head,  development  of  the  tail,  hatching, 
the  external  gills.  What  becomes  of  the  gills?  Do  you  find  any  trace 
of  mouth,  eyes,  nasal  openings?  Where  do  the  legs  first  appear?  What 
becomes  of  the  tail?  Prove.  Tadpoles  of  all  ages  may  usually  be  found 
in  the  shallow  ponds.  These  should  be  compared  with  those  reared 
in  the  laboratory.  Dissect  one  of  the  larger  tadpoles,  and  examine 
particularly  the  intestine  and  the  gill  chamber. 

335.  Compare  with  the  frog  any  other  Amphibian  types  which  can  be 
found, — as  the  toad,  the  newt,  or  the  salamander.  Note  especially  the 
differences  in  habits,  haunts,  external  form,  appendages,  method  and  time 
of  depositing  eggs,  the  form  of  the  tadpoles,  etc. 


CHORDATA.  321 

DESCRIPTIVE  TEXT. 

336.  General  Characters. — In  common  with  the  simpler 
Chordata  thus  far  considered  the  Vertebrata  are  bilaterally 
symmetrical  Metazoa  with  a  coelomic  cavity,  a  notochord  de- 
rived from  the  entoderm,  gill-slits  at  some  stage  of  life,  dorsal 
nerve  tube  and  a  ventral  heart.     In  addition,  the  following 
points  may  be  given  as  distinguishing  the  true  vertebrates : 

1.  The  notochord  comes  to  be  surrounded  by  a  sheath  of  tis- 
sue derived  from  the  mesoderm.     This  produces  around  the 
notochord  the  internal  skeletal  axis,  the  centra  of  the  vertebrae, 
composed  either  of  cartilage  or  bone  (Figs.  154-156). 

2.  Outgrowths  from  these  centra  pass  dorsally  to  protect 
the  nerve  tube,  and  ventrally  to  protect  the  viscera  (Fig.  157). 

3.  Several  sets  of  organs  show  varying  degrees  of  meta- 
meric  segmentation :  e.  g.,  vertebral  column ;  muscular  system ; 
nervous  system. 

4.  Jointed  appendages  having  a  central  skeleton  never  ex- 
ceed two  pairs ;  one  pair  or  both  of  them  may  be  rudimentary 
or  wanting. 

5.  The  respiratory  system  is  developed  in  connection  with 
the  anterior  end  of  the  digestive  tract. 

6.  The  heart  always  has  as  many  as  two  chambers  and  the 
blood  contains  red  corpuscles. 

337.  General  Form. — While  varying  greatly  in  form,  ver- 
tebrates are  typically  elongated  animals  with  the  mouth  at  or 
near  the  anterior  extremity  of  the  long  axis.     The  position 
of  the  anus  is  variable.     It  may  be  one  half  the  length  of  the 
body  from  the  posterior  end.     The  body  is  roughly  divisible 
into  head  and  trunk  with  or  without  an  intervening  neck.    The 
neck  is  more  pronounced  in  the  land  than  in  the  water  forms. 
Posterior  to  the  trunk  containing  the  body  cavity,  there  may 
be  a  tail  into  which  the  skeleton  is  continued  but  which  is 
destitute  of  a  body  cavity. 

Bilateral  symmetry  is  shown  by  the  paired  condition  of  the 
eyes,  ears,  and  other  external  and,  to  a  less  degree,  internal 

22 


322  ZOOLOGY. 

organs.  Metamerism  on  the  contrary  is  much  more  evident 
from  the  internal  than  from  the  external  organs.  There  are 
usually  two  pairs  of  lateral  appendages  for  support  and  loco- 
motion:  the  thoracic  at  the  anterior  end  of  the  trunk,  and  the 
pelvic,  ordinarily  occurring  near  the  union  of  the  trunk  and 
tail.  These  are  variously  modified  as  to  their  form  and  in- 
ternal structure  (e.  g.,  fins,  legs,  arms,  wings),  but  are  looked 
upon  as  homologous.  In  many  water  forms  there  are  median 
appendages  (dorsal,  ventral,  and  caudal  fins)  also  assisting  in 
locomotion.  The  ccelom  or  body  cavity  is  well  represented  in 
the  trunk  region,  and  arises  by  a  splitting  of  the  mesoderm  into 
an  inner  layer  which  comes  to  unite  with  the  digestive  tract 
and  an  outer  layer  which  unites  with  the  ectoderm  (Fig.  154). 
In  this — the  visceral  cavity — beside  the  mesoderm-covered 
digestive  tract  to  which  reference  has  already  been  made,  lie 
the  principal  organs  of  respiration,  of  excretion,  of  circulation, 
and  of  reproduction.  Dorsal  to  the  notochord  the  nervous 
system  occupies  a  cavity  within  the  mesoderm,  which  is  not, 
however,  a  part  of  the  ccelom.  This  is  described  as  the  dorsal 
or  neural  cavity  and  is  protected  by  a  sheath  of  cartilage  or 
bone.  In  the  anterior  region  this  is  much  enlarged  to  accom- 
modate the  brain.  This  condition  of  -a  dorsal  and  ventral 
cavity  is  very  characteristic  of  vertebrates.  In  mammals  the 
ventral  cavity  is  further  divided  by  the  diaphragm  into  an 
anterior  or  thoracic  and  a  posterior  or  abdominal  cavity. 

338.  Protective  and  Supportive  Structures — the  Integu- 
ment.— Covering  the  body  of  vertebrates  is  the  skin,  which 
consists  of  two  layers; — the  outer,  or  epidermis,  which  is  de- 
rived from  the  ectoderm,  and  the  dermis  or  true  skin  which 
is  mesodermal  in  origin.  The  epidermis  consists  of  from  two 
to  many  layers  of  cells  in  thickness,  and  in  the  higher  forms 
the  differentiation  into  layers  becomes  very  pronounced  (Fig. 
153,  E).  The  outermost  cells  of  the  epidermis  frequently  be- 
come hardened  for  the  better  protection  of  the  parts  within. 
This  is  especially  true  of  the  terrestrial  forms.  .The  inner  layer 


CHORDATA. 


323 


of  the  epidermis  is  usually  columnar  in  form,  and  from  this 
.layer  the  outer  cells  are  renewed,  and  all  special  epidermal 
growths  arise  (Fig.  153,  c.e).  The  dermis  consists  largely 


FIG.  153.  Diagram  of  the  skin  in  Mammals,  showing  the  multiple  layered  condition, 
together  with  outgrowths  and  ingrowths.  Drawn  by  Dr.  J.  W.  Folsom.  E,  epidermis; 
D,  dermis;  a,  adipose  tissue, — fat  deposited  amid  the  connective  tissue;  b,  blood  vessels; 
c.e.,  columnar  epithelial  layer  of  the  epidermis;  f,  hair  follicle;  h,  hair;  n,  nerve;  «.<?., 
nerve  ending  (sensory  corpuscle);  p,  pore  of  sweat  gland;  seb.,  sebaceous  or  oil  gland 
of  hair;  s.g.,  sweat  gland;  s.c.,  horny  layer  of  epidermis;  s.m.,  mucous  layer  of 
epidermis. 


Questions  on  the  figure. — What  suggests  that  the  columnar  layer  of 
the  epidermis  is  the  most  vitally  important  layer?  Are  the  hair  and  glands 
dermal  or  epidermal  growths?  Which  structures  found  in  the  dermis 
seem  to  be  invasions  of  that  layer  by  outside  structures?  What  are  the 
functions  of  the  various  layers  of  the  skin  ?  Which  parts  are  ectodermal 
and  which  mesodermal  in  origin  ? 


324  ZOOLOGY. 

of  connective  tissue,  but  contains  in  addition  nerves  and  blood 
vessels  beside  such  ingrowths  from  the  epidermis  as  glands, 
hair-follicles,  etc. 

339.  Special  Products  of  the  Integument  often  occur  in 
the  form  of  outgrowths  or  ingrowths.     Glands  are  examples 
of  the  latter,  and  are  frequent  in  connection  with  the  epidermis. 
They  may  be  simple  and  unicellular  (mucous  glands  in  fishes) 
or  multicellular,  penetrating  deep  into  the  dermis  (sweat  and 
oil  glands,  Fig.  153,  sg).    The  mammary  glands  of  Mammalia 
are  modified  forms  of  the  oil  glands.    The  outgrowths  may  be 
purely  epidermal,  as  in  hair,  feathers,  nails,  hoofs,  claws,  and 
the  scales  of  some  reptiles ;  or  in  other  instances  the  principal 
structures  are  formed  in  the  dermis,  usually  with  an  outer 
layer  contributed  by  the  epidermis,   as   in  the  teeth  or  the 
scales  and  bony  plates  which  form  in  many  instances  (turtle, 
armadillo,  etc.)  a  very  complete  external  skeleton.     Some  of 
the  bones  of  this  external,  or  dermal,  skeleton  persist  even  in 
the  highest  forms  (e.  g.,  man)  and  unite  with  bones  of  the 
internal  skeleton,  as  in  the  formation  of  the  cranium  and  the 
facial  bones. 

The  most  apparent  function  of  the  skin  is  protection.  The 
outgrowths  (hair,  scales,  claws,  etc.)  evidently  increase  its 
adaptation  to  this  function.  In  addition,  the  skin  is  partly 
respiratory  and  excretory.  The  glands  represent  a  specializa- 
tion of  this  latter  function.  It  is  also  sensory,  and  in  an 
indirect  way  assists  in  regulating  bodily  temperature,  espe- 
cially in  the  warm-blooded  types. 

340.  The  Skeleton. — Attention  has  already  been  called  to 
the  exoskeleton  as  the  derivative  of  the  skin.     The  endoskele- 
ton  is  surrounded  by  muscles  separating  it  from  the  integu- 
ment.    In  general  it  may  be  said  that  these  two  bony  systems 
supplement  each  other.    In  the  higher  forms  where  the  internal 
skeleton  is  best  developed  the  exoskeleton  is  usually  reduced 
to  a  minimum.    Elements  from  both  sources  may  become  fused 
in  the  formation  of  a  single  structure  (the  skull;  the  carapace 


CHORDATA. 


325 


of  the  turtle).  A  difference  between  the  internal  and  the 
external  skeleton  is  in  the  fact  that  bone  of  the  former  is 
typically  formed  in  and  around  cartilage,  whereas  in  the  latter 
there  is  no  cartilage.  The  internal  skeleton  consists  of  two 

FIG.  154. 


FTC.  154.  Diagram  of  transverse  section  through  embryo  of  a  Vertebrate,  showing 
the  mode  of  origin  and  the  relations  of  Ihe  notochord,  nervous  cord,  ectoderm,  ento- 
derm  and  mesodeim  (see  also  Fig.  13).  coe.,  coelomic  pouches;  ect.,  ectoderm;  ent., 
entoderm;  g,  lumen  of  the  gut;  iv.,  invagination  of  ectoderm  which  forms  the  nerve 
cord  (see  c,  in  succeeding  figures);  mes1.,  somatic  or  body  mesoderm;  mes2.,  splanchnic 
mesoderm,  that  portion  of  the  mesoderm  which  becomes  allied  with  the  entoderm;  n, 
the  ne.rve  (spinal)  cord;  n.c.,  notochord,  arising  by  an  outpocketing  of  the  entoderm. 

portions,  (i)  the  axial,  embracing  the  vertebral  column,  and 
(2)   the  appendicular,  or  that  supporting  the  appendages. 

341.  Axial  Skeleton. — In  its  simplest  condition  this  con- 
sists of  the  notochord  which  it  will  be  remembered  is  derived 
from  the  entoderm  and  lies  between  the  alimentary  canal  and 
the  spinal  cord  (Fig.  154).  In  the  true  vertebrates,  cells  aris- 
ing from  the  mesodermal  pockets  on  either  side  (Fig.  156) 
produce  a  continuous  skeleton- forming  sheath  about  the  noto- 
chord. From  the  cells  of  this  sheath  are  developed,  finally, 
rings  of  cartilage  or  bone  about  the  notochord  (centrum; 
plural,  centra,  Fig.  157,  c}  and  about  the  spinal  cord  (spinous 
processes,  Fig.  157,  na).  These,  with  certain  other  growths, 
constitute  the  typical  vertebrae.  In  this  process  the  notochord 


326 


ZOOLOGY. 


often  becomes  almost  obliterated  by  the  developing  vertebrae. 
To  each  vertebra  may  be  attached  a  pair  of  ribs,  which  protect 
the  ventral  structures,  somewhat  as  the  neural  arch  protects 
the  nerve  cord.  The  ribs  of  fishes  and  of  the  higher  forms  are 
not  considered  to  be  homologous  structures  (Figs.  1.57,  158). 


FIG.  156. 


;--  CCt. 


--mes'. 


FIG.    155.     Diagram    similar    in    position    and    lettering    to    Fig.    154,    at    a    later    stage. 

c,  central  canal  of  spinal  cord. 

FIG.  156.  Transverse  section  of  an  embryo  Vertebrate  at  a  stage  later  than  Fig.  155. 
m,  mesentery;  sk.,  the  beginning  of  the  mesodermal  skeleton  which  surrounds  the 
notochord  (w.c.),  and  in  part  the  spinal  (nerve)  cord,  n. 

Questions  on  Figs.  154  to  156.— H6w  does  the  mesoderm  originate  in 
vertebrates?  Trace  its  gradual  growth  and  differentiation  in  the  figures. 
What  two  principal  portions  are  to  be  distinguished?  How  does  the 
notochord  arise?  How  the  spinal  cord?  What  is  the  source  of  the  cavity 
of  the  spinal  cord?  From  which  of  the  three  layers  does  the  protecting 
skeleton  arise?  What  does  the  mesentery  connect?  What  other  organs 
might  be  expected  in  the  ccelom,  if  it  were  the  purpose  to  make  a 
complete  diagram  of  the  visceral  organs? 

The  axial  skeleton  varies  from  this  typical  condition  in  dif- 
ferent parts  of  its  course.  In  the  head  region,  for  example, 
the  nervous  cord  is  immensely  enlarged  and  the  neural  arches 
are  much  modified,  being  replaced  by  plates  and  supplemented 
by  the  dermal  bones.  The  following  regions  may  be  described 
as  typical : 


CHORDATA. 


327 


1.  Head  region    (skull)    embracing  the  cranium  or  brain 
case  and  its  associated  ventral  arches  including  the  bones  of 
the  face. 

2.  Cervical  vertebrae,  located  in  the  neck  and  lacking  ribs. 
Usually  the  anterior  one  or  two  are  considerably  modified. 

FIG.  157. 


-n.s. 


FIG.  157.  Diagram  of  vertebrae  of  a  bony  fish.  A,  caudal;  B,  trunk,  c,  centrum  or 
body  of  the  vertebra;  ch.,  the  notochord;  h.a.,  haemal  arch;  h.c.,  haemal  canal;  h.s., 
haemal  spine;  h.s.,  haemal  zygapophysis,  or  articulating  facet;  m.b.,  inter-muscular 
bone;  n.a.,  neural  arch;  n.c.,  neural  canal;  n.s.,  neural  spine;  n.z.,  neural  zygapophysis; 
r,  rib. 

Questions  on  the  figure. — What  is  the  meaning  of  haemal  ?  Of  neural  ? 
In  life  what  occupies  the  neural  canal  ?  What  occupies  the  haemal  canal  in 
the  caudal  region  ?  In  the  trunk  region  ?  Is  there  anything  to  suggest  that 
the  ribs  in  fishes  are  homologous  with  tl^eprocesses  which  form  the 
haemal  arch  (h.  a.)  ?  ^fcl 

3.  Dorsal  vertebrae,  in  the  thoracic  region  and  bearing  well- 
developed   ribs   which   may  unite   with   a   ventral  bone,   the 
sternum. 

4.  Lumbar  vertebrae,  following  the  dorsal  vertebrae  and  not 
bearing  ribs. 

5.  Sacral  vertebrae,  usually  a  small  number  of  vertebrae,  fre- 
quently fused  into  one  piece  with  which  the  girdles  of  the 
posterior  appendages  unite. 

6.  Caudal  vertebrae,  posterior  to  the  sacrum  and  possessing 
no  ribs. 


3 2$  ZOOLOGY. 

The  number  of  bones  in  these  regions  is  very  variable  in 
the  phylum  as  a  whole,  but,  in  the  higher  forms  particularly, 
individuals  of  related  species  present  remarkable  uniformity. 

(The  discussion  of  the  condition  of  the  skull  and  the  origin  of  its 
parts  is  entirely  too  technical  for  an  elementary  text.  The  student  should 
be  referred  to  more  advanced  works.) 

342.  The  Appendicular  Skeleton. — Here  are  embraced 
the  skeletal  parts  of  the  appendages  proper,  together  with  the 
bones  binding  them  to  the  axial  skeleton  (girdles).  Each 
girdle  may  be  said  to  consist  typically  of  three  bones,  uniting 

FIG.  158. 


FIG.  158.  Diagram  of  a  trunk  vertebra  in  a  Mammal,  c,  centrum;  ch.,  position 
originally  occupied  by  the  notochord;  h.,  head  of  the  rib;  /i.e.,  haemal  cavity;  n.a., 
neural  arch;  n.c.,  neural  canal;  n.s.,  neural  spine;  r.,  rib;  st.,  sternum;  s.c.,  sternal 
cartilage  uniting  ribs  and  sternum;  t.p.,  transverse  process  of  vertebra;  tu.,  tubercle 
of  rib. 

Questions  on  the  figure. — Compare  all  the  parts  here  with  corre- 
sponding ones  of  Figs.  157 :  A,  B,  and  note  the  differences.  What  is  gained 
by  the  articulation  of  ribs  with  a  sternum?  What  is  lost?  In  which 
groups  of  Vertebrates  is  a  sternum  found?  In  which  are  fully  developed 
ribs  found? 

to  form  a  joint  with  the  first  bone  of  the  limb.  One  of  these  is 
dorsal  and  the  others  ventral  (Fig.  159,  B;  il,  is,  p.).  The 
appendages  are  much  alike  both  as  to  their  girdles  and  the 
limbs  proper.  The  posterior  is,  in  higher  forms,  more  inti- 
mately fused  with  the  axial  skeleton,  thus  securing  greater 


CHORD  AT  A. 
FIG.  159. 


329 


ft.-  — 


U-J  tl. 


kn.  k 


-~.fi. 


FIG.  159.  Diagrams  of  the  girdles  and  appendages  in  a  typical  Vertebrate.  A, 
anterior;  B,  posterior,  ac.,  acetabulum,  articulation  of  the  humerus  with  its  girdle; 
c,  coracoid;  ca.,  carpals;  ce.,  centralia;  d.c.,  distal  carpals;  d.t.,  distal  tarsals;  el., 
elbow  joint;  f,  fibula;  fe.,  femur;  fi.,  fibulare;  g.c.,  glenoid  cavity,  articulation  of  arm 
with  girdle;  h,  humerus;  il.,  ilium;  in.,  intermediale ;  is.,  ischium;  kn.,  knee  joint;  m.c., 
metacarpals  (1-5);  m.t.,  metatarsals  (1-5);  P,  pubis;  ph.,  phalanges  (1-5);  pr.c.,  pre- 
coracoid;  r,  radius;  ra.,  radiale;  sc.,  scapula;  t,  tibia;  ta.,  tarsals;  ti.,  tibiale;  »., 
ulna;  ul.,  ulnare. 


33°  ZOOLOGY. 

Questions  on  the  figure. — Compare  the  two  appendages  throughout 
and  note  the  corresponding  bones.  How  much  is  girdle?  How  much 
appendage  proper?  How  many  carpals?  Tarsals?  Which  are  proximal? 
Which  distal?  How  do  the  phalanges  differ?  Which  is  the  thumb?  How 
can  you  be  sure?  Compare  this  figure  with  figures  (in  reference  texts) 
of  the  appendages,  botn  front  and  rear,  of  the  frog;  of  some  bird;  of 
some  Carnivore ;  of  the  horse ;  of  man.  Where  are  the  greatest  varia- 
tions, i.  c.,  which  bones  depart  most  from  this  typical  condition? 

strength  at  the  expense  of  freedom  of  motion.  The  first  joint 
of  each  appendage  consists  of  one  bone  (arm  or  thigh)  ;  the 
second,  of  two  (forearm  or  shank)  ;  then  follows  a  region  of 
several  small  bones  (wrist  or  ankle),  succeeded  by  the  hand 
or  foot  with  five  (usually)  bones,  and  then  by  five  digits 
(fingers,  toes)  of  a  varying  number  of  joints.  The  accom- 
panying diagrams  (Fig.  159)  will  make  clear  these  relations,  as 
well  as  the  names  of  the  bones.  Bones  may  disappear  or  fuse 
with  others  in  such  a  way  as  to  cause  a  wide  variation  from 
this  type;  indeed  the  type  is  perhaps  never  realized  in  any 
single  animal.  In  fishes  the  appendage  and  the  girdle  are 
often  very  simple,  the  limb  being  little  more  than  radiating 
fin-rays  covered  by  a  membrane  (Figs.  174,  175).  Yet  it  is 
believed  that  from  some  such  primitive  condition  the  more 
specialized  appendages  have  arisen. 

343.  The  Digestive  Organs. — As  in  many  of  the  inverte- 
brates which  we  have  studied,  the  alimentary  canal  in  the 
vertebrates  possesses  an  anterior,  ectodermal  portion  (stomod- 
seum),  a  mid-gut  lined  with  entoderm  (mesenteron),  and  a 
posterior  ectodermal  part  (proctodseum).  The  tract  is  lined 
throughout  with  a  mucous  membrane.  Outside  of  this  are  the 
layers  of  unstriped  muscle  fjbres,  circular  and  longitudinal, 
by  which  the  food  is  forced  onward.  The  muscles  are  especi- 
ally developed  in  certain  regions,  as  in  the  stomach.  Outside 
of  all  these,  in  the  portion  passing  through  the  body  cavity,  is 
the  serous  membrane  derived  from  the  mesoderm,  a  portion 
of  the  lining  of  the  body  cavity.  The  mucous  surface  which 
is,  naturally  enough,  the  important  portion  in  digestion  and 
absorption  may  be  increased  by  the  lengthening  of  the  tube  as 


CHORDATA.  331 

a  whole  or  by  means  of  outgrowths  (the  glands)  or  by  in- 
growths (folds  of  various  kinds).  The  highly  nourished  con- 
dition of  the  entodermal  sheet  of  cells  presumably  leads  to 
their  rapid  growth  and  foldings.  The  folds  are  often  so 
arranged  across  the  axis  of  the  tube  as  to  retard  the  progress 
of  the  food  through  the  tract,  thus  making  digestion  and 
absorption  more  complete,  by  increasing  the  time  during  which 
the  food  is  exposed  to  the  action  of  the  digestive  juices,  and 
to  the  absorbing  surface. 

344.  The  Divisions  of  the  Tract. — The  mouth,  which  may 
be  either  terminal  or  ventral,  opens  into  the  buccal  cavity, ' 
which  is  bounded  dorsally  by  the  floor  of  the  brain  case,  on  the 
sides  and  in  front  by  the  jaws,  and  ventrally  by  a  muscular 
floor  from  which  the  tongue  arises  as  a  fold.  The  jaws  are 
made  up  of  bony  elements  from  two  sources;  a  core  of  bones 
from  the  internal  skeleton  (the  first  visceral  arch)  and  a 
covering  of  dermal  bones  which  fuse  with  it.  The  latter  are 
the  bones  which  (typically)  bear  the  teeth.  Teeth  however 
occur  in  the  lower  vertebrates  in  the  roof  of  the  mouth  or  on 
the  tongue.  Their  place  may  be  taken,  by  horny  epidermal 
structures,  as  in  the  beak  of  birds.  When  present  the  salivary 
glands  open  into  the  mouth  cavity.  Posteriorly  the  buccal 
cavity  communicates  with  the  pharynx,  which  may  be  defined 


FIG.    1 60.     Stomach  of  Dog   (A)   and  of   Rat   (5).     c,  cardiac   portion;   p,  pyloric 
'tion;    o,    oesophagus;    i,    intestine. 


332 


ZOOLOGY. 

FIG.  161. 


FIG.  161.  Diagram  of  the  stomach  of  a  ruminant,  o,  oesophagus;  r,  rumen  or  paunch; 
re.,  reticulum,  or  honeycomb;  p,  psalterium  or  manyplies;  a,  abomasum  or  rennet; 
i,  intestine. 

Questions  on  Figs.  160  and  161. — Taken  as  a  series,  what  is  illustrated 
by  the  three  diagrams?  What  do  the  arrows  indicate?  What  is  known 
of  the  function  of  the  various  portions  of  the  ruminant  stomach? 

as  the  part  of  the  digestive  tract  in  connection  with  which  the 
lungs  or  gills  are  developed.  In  fishes  and  in  the  embryos  of 
higher  forms  there  are  slits  in  the  side  walls  connecting  the 
pharynx  with  the  outside.  Gills  are  developed  in  the  walls 
of  these  slits.  In  forms  above  fishes  the  slits  become  closed  as 
the  embryo  develops.  Above  the  Amphibia  they  never  bear 
gills. 

The  oesophagus  is  a  narrow  muscular  tube  of  varying  length 
leading  to  the  stomach.  In  birds  an  enlarged  portion  of  it 
(the  crop)  may  serve  for  the  temporary  storing  and  softening 
of  the  food. 

The  stomach  is  usually  well  differentiated  and  may  consist 
of  one  chamber  or  of  several.  In  the  latter  case  there  is  a 
division  of  labor  among  the  parts.  One  portion  may  be  highly 
muscular  and  supplied  with  a  hardened  internal  lining  for 
grinding  the  food  (gizzard  of  fowls,  Fig.  162)  ;  in  such  in- 
stances another  portion  is  glandular.  In  the  ruminants  (ox. 


CHORDATA. 


333 


deer,  etc.)  there  are  four  chambers  in  the  stomach  (Fig.  161). 
The  gastric  glands  produce  an  acid  secretion  which  contains  a 
ferment  acting  chiefly  on  proteid  foods. 

The  food  is  retained  in  the  stomach  by  means  of  a  circular 
(sphincter)  muscle  at  its  posterior  end  where  it  narrows  into 
the  intestine.  This  latter  portion  is  the  principal  digestive  and 
absorptive  portion  of  the  tract  and  varies  much  in  length  in 

FIG.  162. 


FIG.  162.  Diagram  of  the  stomach  and  oesophagus  of  the  Fowl,  o,  oesophagus;  c, 
crop;  p,  proventriculus  or  glandular  stomach;  g,  gizzard  or  grinding  stomach;  i, 
intestine. 

Questions  on  the  figure. — Compare  this  figure  with  that  of  the 
stomach  of  ruminants  as  to  complexity.  What  are  the  functions  of  the 
various  portions?  What  changes  take  place  in  the  gizzard  of  flesh-eating 
birds  if  they  are  forced  to  live  on  grain?  Why  is  the  crop  located  outside 
the  cavity  inclosed  by  the  ribs? 

the  various  groups  in  accordance  with  the  nature  of  the  food 
used, — the  vegetable  feeders  for  the  most  part  possessing  the 
longest  intestines.  Numerous  circular  or  spiral  folds  of  the 
mucous  membrane  occur  in  the  intestine.  Special  absorptive 
organs  (villi)  supplied  with  blood  and  lymph  vessels  may 
cover  these  folds.  Near  the  anterior  end  the  ducts  of  the  liver 
and  pancreas  open  into  the  intestine.  The  liver  is  the  largest 
of  the  glands,  and  the  pancreas  one  of  the  most  important  in 
digestion.  The  intestine  may  open  directly  on  the  exterior 
(most  mammals),  or  into  ah  epidermal  pocket  (cloaca)  which 


334  ZOOLOGY. 

also  receives  the  excretory  and  genital  products  (reptiles  and 
birds). 

345.  Exercises  for  Field  and  Library. 

1.  What   differences   have  you   observed   in   the  number,   position,   and 
kinds  of  teeth  in  the  various  vertebrates  of  your  acquaintance? 

2.  Can  you  cite  from  your  observation  any  evidences  of  adaptation  of  the 
digestive  tract  to  the  peculiar  food  and  habits  of  the  animal  possessing  it? 
Supplement  by  library  references. 

3.  To  what  extent  is  food  prepared  in  the  mouth,  i.  e..  antecedent  to 
swallowing,  in  the  various  vertebrates  whose  habits  you  have  observed? 

346.  Respiration. — As  in  all  higher  animals  there  are  two 
things  to  be  considered  in  the  respiration  of  vertebrates :  ( I )' 
the  exchange,  between  the  blood  and  the  external  medium, 
air  or  water,  of  carbon  dioxid   for  oxygen,  which  may  be 
called  external  respiration,  and   (2)   the  exchange  by  which 
the  blood  gives  the  cells  of  the  body  oxygen  and  receives  their 
carbon  dioxid,  or  internal  respiration.  -   The  former  is  usu- 
ally meant  when  the  simple  term  respiration  is  used,  though 
the  latter  is  really  the  vital  process.     A  certain  amount  of 
respiration  takes  place  through  the  skin  in  almost  all  verte- 
brates.   Beside  this,  special  devices — both  gills  and  lungs — are 
developed  by  which  the  blood  and  the  medium  containing  the 
oxygen  are  brought  into  closer  relation.     In  fishes  and  larval 
amphibians  gills  are  present ;  in  most  adult  amphibians  and  in 
reptiles,  birds,  and  mammals,  only  lungs  occur. 

Gills  are  thin-walled  external  folds  or  groups  of  filaments 
bounded  by  a  mucous  membrane,  in  which  the  blood  circulates 
freely.  In  vertebrates  they  are  found  on  the  wall  of  passages 
leading  from  the  pharynx  to  the  outside  (gill-slits).  The 
water  passes  into  the  mouth  and  out  over  the  gills,  through  the 
thin  walls  of  which  the  gases  are  exchanged.  The  walls  be- 
tween the  slits  may  be  supported  by  cartilages  or  bones 
(visceral  or  gill-arches}.  The  gill-slits  vary  from  four  to 
eight  in  number.  In  the  higher,  air-breathing  vertebrates 
traces  of  the  gill-slits  appear  in  embryonic  development,  but 
they  never  bear  gills.  (See  Figs.  30,  31.) 


CHORDATA. 


335 


347.  Lungs  arise  as  outpocketings  of  the  ventral  wall  of 
the  pharynx.     These  may  persist  as  relatively  simple  sacs,  as 
in  the  frog,  or  by  great  growth  and  folding  they  may  become 
very  complicated,  and  thus  increase  their  surface  to  a  wonder- 
ful degree.     They  are  lined  throughout  with  the  entodermal 
epithelium.      The  blood   capillaries   are   in  contact   with   this 
layer  and  through  these  thin  walls  the  gases  are  exchanged. 
The  outer  surface  of  the  lung  is  covered  by  the  pleura,  the 
lining  of  the  body  cavity.     The  tube  connecting  the  pharynx 
with  the  body  of  the  lung  is  known  as  the  trachea.    The  upper 
or  anterior  end  of  the  trachea  is  modified  into  a  chamber 
known  as  the  larynx  in  the  air-breathing  vertebrates.     The 
epiglottis   closes    the    opening    (glottis)    from    the    pharynx 
into  the  larynx,  whenever  food  is  passing  from  the  mouth 
through   the  pharynx   into'  the   gullet.      On   account  of  the 
presence  of  currents  of  air  passing  in  and  out  and  capable  of 
producing  vibration,  certain  portions  of  the  tract  are  used  in 
making  definite  sounds  whereby  the  animals  are  put  into  com- 
munication with  their  kind.     The  parts  so  used  are  the  lips, 
teeth  and  vocal  cords.     The  latter  are  membranous  folds  in 
the  mucous  lining  of  the  larynx,  which  may  be  brought  into 
such  a  position  as  to  close  that  organ,  in  part,  to  the  escaping 
current  of  air.    The  tense  edges  of  the  membrane  are  put  into 
vibration.    The  resulting  sound,  reinforced  or  otherwise  modi- 
fied by  the  other  organs,  is  voice. 

348.  Supplementary  Exercises  for  Library. — Where  does 
the  "swim-bladder"  in  fishes  occur?   Is  any  thing  known  of 
its  function  ?  Is  any  thing  like  a  lung  known  among  the  fishes  ? 

What  are  the  most  important  differences  between  the  "  voice- 
box  "  of  mammals  and  that  of  birds?  Have  all  the  vertebrate 
groups  vocal  organs?  Do  they  all  have  voice?  What  is  the 
difference  between  voice  and  speech?  What  are  the  uses  of 
voice  to  animals  possessing  it  ? 

349.  Circulation. — The  blood  in  vertebrates  contains  both 
colorless   and   colored    (red)    corpuscles      The   red   coloring 


336 


ZOOLOGY. 


matter  (haemoglobin)  has  an  affinity  for  oxygen  and  thus  be- 
comes a  vehicle  for  transporting  it.  The  colored  corpuscles 
have  no  motion  of  their  own,  but  are  carried  by  the  blood  cur- 
rents. The  colorless  cells  are  much  less  numerous  than  the  red 

FIG.  163. 


S.  V. 

FIG.  163.  Diagrams  of  the  structure  of  the  heart  in  the  lower  Vertebrates.  A, 
primitive  condition;  B,  the  position  of  the  parts  in  the  fishes,  a,  artery;  au.,  auricle; 
c,  conus  arteriosus  with  valves;  s.v.,  sinus  venosus;  v,  valves;  ve.,  vein;  vent., 
ventricle.  The  dorsal  portion  of  the  heart  is  toward  the  bottom  of  the  figure. 

Questions  on  the  figures. — Which  side  of  the  figure  represents  the 
anterior?  Compare  the  walls  of  the  vessels.  Where  are  the  valves  lo- 
cated? How  is  the  term  "  sigmoid  flexure"  appropriate  to  the  form  in  Bf 
Notice  how  it  results  in  what  is  morphologically  the  posterior  portion  of 
the  heart  becoming  anterior.  Trace  the  course  of  the  flow  of  the  blood. 

and  have  power  of  independent  motion  (amoeboid).  The  fluid 
in  which  the  cells  float  is  called  the  plasma  and  carries  the 
food  and  waste  materials  of  the  body  in  solution. 

The  muscular  heart  always  consists  of  at  least  two  cham- 
bers, ( i )  an  auricle  which  receives  blood  f rorn  the  veins,  and 
(2)  a  ventricle  which  has  thick  walls  and  propels  the  blood 
into  the  arteries.  Morphologically  the  auricle  is  the  posterior 
portion  of  the  heart  (Fig.  163,  A ) ,  but  in  development  the  heart 
has  undergone  an  ^--shaped  bending  which  has  brought  the 
auricle  in  front  of  the  ventricle  (Fig.  163,  B).  The  veins  and 
arteries  are  often  specially  enlarged  and  modified  in  the  region 


CHORDATA. 


337 


of  the  heart.  The  main  trunk  leaving  the  heart  is  called  the 
aorta.  As  the  vessels  are  followed  from  the  heart  they  branch 
and  become  smaller  and  the  walls  become  thinner.  The  final 
divisions  are  the  capillaries  through  the  thin  walls  of  which 


FIG.  164. 


FIG.  165. 


.  v.  r. 


a.  a. 


l-c.  v.  I. 


c.  v.  I. 


p.c. 


FIG.  164.  Diagram  of  the  heart,  the  branchial  arches,  and  the  principal  veins  in  the 
Teleosts.  Ventral  view.  The  heart  is  represented  without  the  sigmoid  flexure;  that  is, 
with  the  auricle  posterior.  The  same  is  true  of  Figs.  165  to  169.  a,  aorta;  au.,  auricle; 
br.a.,  branchial  arches  of  the  aorta  (1-4,  numbering  from  the  front);  c,  carotid;  c.v., 
cardinal  veins  (right  and  left);  d.a.,  dorsal  arteries;  ;',  jugular  veins;  d.c.,  ductus 
Cuvieri;  s.i'.,  sinus  venosus;  i',  ventricle. 

Questions  on  the  figure. — Refer  to  the  table  on  page  340  and  identify 
the  parts  there  described.  Compare  this  figure  with  those  following 
(Figs.  165-169).  Compare  also  with  Figs.  178  and  179,  Ch.  XX.  Which 
is  the  anterior  and  which  the  posterior  portion  of  this  and  the  follow- 
ing figures  ? 

FIG.  165.  Diagram  of  heart  and  branchial  arches  in  Ceratodus  (one  of  the  Dipnoi). 
Position  and  lettering  as  in  Fig.  164.  a.b.,  air  bladder  (lung);  p. a.,  pulmonary  artery; 
p.c.,  post  caval  vein  (right)  ;  p.v.,  pulmonary  vein. 

Questions  on  the  figure. — What  organs  appear  in  this  diagram  which 
are  not  present  in  Fig.  164?  What  changes  of  the  various  portions  do  you 
note  in  comparing  the  two  figures?  • 

23 


338 


ZOOLOGY. 


the  blood  exchanges  materials  with  the  tissues  (Fig.  32,  c.s; 
c.r) .  The  capillaries  unite  to  form  the  smaller  veins  and  these 
uniting,  complete  the  circuit  back  to  the  heart.  It  is  evident 
that  the  capillaries  are  the  most  important  portion  of  the  sys- 
tem, the  part  for  which  the  rest  in  reality  exists. 


FIG.  1 66. 


FIG.  167. 


pre.  c. 


FIG.  166.  Diagram  of  the  heart  and  branchial  arches  in  Protopterus  (one  of  the 
Dipnoi).  Position  and  lettering  as  in  the  preceding,  pre.c.,  precaval  vein,  made  up 
of  right  and  left  jugulars,  subclavians,  etc.;  post.c.,  postcaval,  made  up  of  the  cardinals, 
right  and  left. 

Questions  on  the  figure. — What  are  the  chief  differences  between  the 
conditions  here  and  in  the  preceding  figures:  (i)  as  to  the  heart:  (2)  as 
to  arteries;  (3)  as  to  veins;  (4)  as  to  lungs? 

FIG.    167.     Diagram  of  the  heart  and  branchial  arches  in  the  Frog,     e.g.,  carotid  gland; 
I,  lungs;  I. a.,  left  auricle;  r.a.,  right   auricle. 

Questions  on  the  figure. — How  does  the  heart  differ  from  that  of  the 
Dipnoi?  How  many  branchial  arches  of  the  aorta  are  shown?  What 
evidences  can  you  find  by  comparison  that  the  pulmonary  arch  is  derived 
from  the  3d  or  4th  branchial?  What  evidences  that  the  carotid  and  sys- 
temic are  the  first  and  second  respectively?  Compare  with  the  table  on 
page  340.  Is  there  anything  to  indicate  that  the  impurest  blood  in  the 
heart  will  go  to  the  lungs? 


CHORDATA. 


339 


350.  In  Figs.  164  to  169  will  be  found  diagrams  of  the 
circulation  in  the  principal  groups  of  vertebrates.  It  will  be 
seen  that  there  is  a  progressive  complication  of  the  structure, 
involving  the  heart,  veins,  and  arteries,  as  we  ascend  the  scale. 

FIG.  168.  FIG.  169. 

-C.  li—  c. 


MT.C 


O/. 


FIG.   1 68.     Diagram    of    the    heart    and    branchial    arches    in    a    Reptile.     Position    and 
lettering  as  in  preceding  figures,     l.v.,   left  venticle;   r.v.,   right  venticle. 

Questions  on  the  figure. — Compare  this  with  figures  164-167  and  make 
a  note  of  the  differences.  How  much  communication  is  there  between  the 
two  sides  of  the  heart?  What  tends  to  insure  that  the  purest  blood  in  the 
heart  shall  go  to  the  head?  That  the  least  pure  goes  to  the  lungs? 

FIG.  169.  Diagram  of  the  heart  and  .the  branchial  arqhes  in  Mammals.  A  dotted 
outline  of  the  arches  of  the  Fish  is  drawn  for  ready  comparison.  The  auricles  are 
represented  in  a  posterior  position,  as  in  the  preceding  figures. 

Questions  on  the  figure. — What  changes  in  the  heart  are  shown  in 
this  as  compared  with  former  figures?  In  the  systemic  branchial  arch? 
Remember  that  the  heart  is  not  represented  in  its  normal  position;  the 
auricles  are  really  at  the  anterior  of  the  heart  (see  Fig.  164).  Compare 
this  condition  with  table,  page  340.  What  are  the  grounds  for  believing 
that  the  auricles  are,  morphologically,  the  posterior  part  of  the  heart? 


340 


ZOOLOGY. 


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CHORDATA.  34! 

These  changes  accompany  and  are  partly  caused  by  the  change 
from  gills  to  lungs.  See  also  the  accompanying  table.  Locate 
the  vessels  and  trace  the  changes  indicated  by  means  of  larger 
texts. 

351.  In  fishes  the  blood  passes  through  the  heart  only  once 
in  making  the  circuit  of  the  body.     In  all  the  air-breathing 
forms  at  least  a  part  of  it  returns  twice,  passing  from  the 
heart  to  the  lung,  then  back  to  the  heart,  and  thence  to  the 
system  and  back  to  the  heart  again.     In  amphibia  and  reptiles 
the  blood  from  the  lung  and  from  the  system  mix  somewhat 
in  the  heart,  because  the  partition  between  the  right  and  left 
sides  is  not  complete,  but  in  birds  and  mammals  the  two  sides 
of  the  heart  are  completely  separated  and  the  pure  and  impure 
blood  are  not  allowed  to  mix  (see  Figs.  167,  169). 

352.  Excretion. — We  have  seen  that  carbon  dioxid,  one 
of  the  waste  products  of  the  protoplasmic  activity,  is  eliminated 
through  the  lungs  and  skin.    Water  is  similarly  excreted.    The 
most  important  remaining  waste  (e.  g.,  urea)  contains  nitro- 
gen.    This  is  taken  from  the  blood  by  means  of  the  kidneys, 
a  pair  of  organs  very  complicated  both  as  to  structure  and 
development.     They  lie  near  the  middle  line  of  the  body  at 
the  back  of  the  body  cavity.     Each  gland  represents  a  large 
number  of  nephridia  or  tubules  similar  in  some  respects  to  the 
segmental  organs  of  worms  (Fig.  33),  but  much  more  com- 
plicated.    The  kidneys  are  always  well  supplied  with  blood. 
In  fishes,  amphibians,  reptiles,  and  birds  both  arteries  and  veins 
carry  blood  to  the  kidney;  in  mammals,  only  arteries.     The 
excretion,  more  or  less  in  solution  in  water,  is  poured  by  the 
tubules  into  a  duct — the  ureter — which  may  be  the  final  outlet ; 
or  the  ureters  may  empty  first  into  a  urinary  bladder,  which 
has  its  own  outlet  ( the  urethra ) . 

353.  Reproduction. — With  a  very  few  exceptions  among 
the  fishes  the  sexes  are  separate  in  all  the  vertebrates.     The 
sexual  elements  are  derived   from  modified  portions  of  the 
lining  of   the   body-cavity    (germinative   epithelium).     This 


342  ZOOLOGY. 

layer,  supported  by  connective  tissue,  forms  the  essential  part 
of  the  ovaries  and  testes,  of  which  there  is  usually  a  single  pair. 
The  eggs  vary  in  size  from  T(yT  of  an  inch  in  mammals 
to  5  inches  (ostrich),  or  more  in  some  extinct  birds. 
The  outlets  for  the  ova  and  spermatozoa  (oviducts 
and  vasa  deferentia)  are  modified  portions  of  the  em- 
bryonic excretory  and  kidney  ducts.  Throughout  the  group 
there  is  a  close  connection  between  the  excretory  and  the  re- 
productive organs.  The  oviducts  may  have  special  glands  for 
depositing  nutritive  or  protective  material  about  the  egg  be- 
fore or  after  fertilization  (as  the  albumen  and  shell  in  egg 
of  birds).  Fertilization  is  external  in  most  fishes  and  some 
amphibia,  and  internal  in  the  higher  groups.  The  uterus  is  a 
special  portion  of  the  oviduct  where  early  embryonic  develop- 
ment may  occur.  (See  Figs.  202,  203.) 

354.  Development. — Those  eggs  which  are  fertilized  out- 
side develop  principally  by  means  of  the  yolk  of  the  ovum. 
Those    internally    fertilized    may    receive,    after    impregna- 
tion, additional  materials  for  the  further  nourishment  of  the 
embryo,  as  above  noticed  for  reptiles  and  birds.    The  fertilized 
ova  may  be  retained  for  a  longer  or  shorter  time  in  the  ovi- 
duct or  in  some  modified  portion  of  it  (uterus,  in  mammals) 
and  undergo  development  there.    Where  this  internal  develop- 
ment is   slight    (as   in  birds)    the  animals  are  described  as 
oviparous;  where  it  is  considerable,  as  in  mammals,  and  the 
young  are  free  at  birth  they  are  described  as  viviparous. 

The  table  on  page  343  will  give  some  of  the  facts  concerning  the  early 
development  of  vertebrates.  It  will  be  found  an  excellent  exercise  to 
have  students  verify  the  data  collected  in  this  and  the  preceding  table. 
The  teacher  can  readily  supplement  it  by  a  demonstration  of  figures  from 
more  advanced  texts. 

355.  The  Muscular  System. — We  have  seen  above  (§  337) 
that  the  internal  layer  of  the  mesodermic  pockets  comes  to 
be  united  with  the  digestive  tract  and  furnishes  the  non-striped 
muscle  fibres  of  its  walls.     The  external  portion,  which  be- 
comes associated  with  the  ectoderm,  gives  rise  to  the  muscles 


CHORD  AT  A. 


343 


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344  ZOOLOGY; 

of  the  body-wall  and  those  which  move  the  skeleton.  The 
fibres  of  these  muscles  are  cross-striped  or  voluntary  (Fig. 
28).  It  is  by  means  of  them  that  locomotion  is  effected.  The 
skeletal  muscles  may  be  classed  as  (i)  axial,  and  (2)  appendi- 
cular.  The  axial  are  well  shown  in  Amphioxus  (Fig.  152)  and 
the  fishes,  where  the  whole  body  is  made  up  of  repeated  seg- 
ments (myotomes)  of  muscle  fibres.  The  muscle  segments 
alternate  with  the  segments  of  the  spinal  column,  as  one  would 
expect.  The  appendicular  muscles  are  those  which  move  the 
limbs.  Their  general  arrangement  will  be  seen  from  the  study 
of  the  frog.  In  the  higher  vertebrates  the  segmentation  of  the 
axial  muscles  becomes  less  conspicuous  especially  in  the  head 
region,  and  the  appendicular  muscles  become  relatively  of 
greater  importance  because  of  the  greater  use  of  the  append- 
ages. 

356.  The  Nervous  System. — The  nervous  system  in  ver- 
tebrates consists  of  two  portions,  the  central  and  peripheral. 
The  central  nervous  part  embraces  the  deep-seated  organs,  the 
brain  and  spinal  cord,  and  has  for  its  most  characteristic  fea- 
ture numerous  ganglion  cells.     From  these  central  cells  the 
cell-processes  or  fibres  pass  to  the  various  tissues  of  the  body, 
terminating    in    a    manner    appropriate    to    the    special    case 
whether  it  be  a  muscle,  sense  organ,  or  gland.     These  nerves 
and  their  endings  constitute  the  peripheral  part  of  the  system. 

357.  The  Central  Organs. — The  central  nervous  system 
originates   from  the  ectoderm  as  a  longitudinal  groove-like 
depression  in  the  mid-dorsal  line  of  the  embryo.     The  union 

'of  the  edges  of  this  fold  produces  a  tube  and  an  overgrowth  of 
the  ectoderm  separates  it  from  the  outside  world  (Fig.  154). 
It  becomes  surrounded  by  mesodermal  elements  (bone  and 
connective  tissue,  Fig.  156),  and  itself  undergoes  numerous  and 
complex  changes.  At  the  anterior  end  of  the  tube  occur  three 
distinct  enlargements  (Fig.  170,  A).  These  are  known  as  the 
primary  vesicles  of  the  brain,  and  by  the  later  growth  and 
differentiation  of  their  walls  they  give  rise  to  the  five  brain- 


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FIG.  170.  Diagrams  of  the  brain  in  Vertebrates,  showing  the  primitive  regions  and 
their  chief  modifications.  A,  early  stages,  showing  the  anterior  enlargement  in  three 
lobes:  I,  II,  and  III,  the  primary  vesicles.  B,  a  sagittal  section,  showing  the  more 
fundamental  modifications  of  the  walls  of  the  primary  brain  vesicles.  C,  frontal  section 
of  same.  D,  lateral  view  of  the  brain  of  the  Frog.  The  vertical  lines  indicate 
corresponding  points  in  the  different  diagrams,  i,  2,  3,  and  4  are  the  ventricles  of  the 
brain,  c.c.,  crura  cerebri;  cer.,  cerebellum  (hind  brain);  h,  hemispheres  (cerebrum  or 


346  ZOOLOGY. 

fore  brain);  inf.,  infundibulum ;  med.,  medulla  oblongata;  mcs.,  mesencephalon  or  mid 
brain;  met.,  metencephalon  or  hind  brain;  my.,  myencephalon  or  medulla;  n1,  olfactory 
nerve;  n-,  optic  nerve;  ol.,  olfactory  lobe;  op.,  optic  lobes;  o.th.,  optic  thalamus;  pr., 
prosencephalori,  or  fore  brain;  p.v.,  pons  Varolii;  s.c.,  spinal  cord;  th.,  thalamen- 
cephalon  or  "  between  "  brain. 

Questions  on  the  figures. — What  portions,  of  the  adult  brain  are  pro- 
duced from  each  of  the  three  primary  lobes?  Where  are  the  principal  out- 
growths, thickenings  and  thin  portions  of  the  wall?  In  comparison  with 
figure  D  what  portions  of  the  brain  become  highly  developed  in  the 
higher  Vertebrates?  Make  a  diagram  based  on  D,  which  will  show  the 
general  relation  of  these  parts  in  man.  Compare  the  diagrams  with  the 
table  on  page  347,  and  verify  the  statements  there. 

regions  of  the  adult.  The  brain  must  be  considered  merely  as 
the  specially  modified  anterior  portion  of  the  spinal  cord. 

Three  sets  of  changes  occur  in  the  development  ri  the  adult 
vertebrate  brain  from  this  primitive  condition : 

T.  The  axis  becomes  more  or  less  curved,  the  concavity 
being  ventral. 

2.  Outpocketings   of   the   walls   occur,,  in   special   regions, 
whose  cavities  (ventricles)  retain  connection  with  the  central 
cavity  (e.  g.,  the  hemispheres).     See  Fig.  170,  h,  pi. 

3.  Thickenings  or  thinnings  of  the  roof,  sides,  or  floor  of 
the  tube  may  produce  lobes  and  affect  the  size  of  the  cavity  of 
the  tube.     The  accompanying  diagrams  and  table  will  furnish 
an  outline  from  which  the  teacher  may,  if  he  desire,  pursue 
the  details  somewhat  further. 

359.  That  portion  of  the  central  nervous  system  not  en- 
closed in  the  skull  is  called  the  spinal  cord.  It  is  surrounded 
and  protected  by  the  dorsal  arches  of  the  vertebrae.  The 
cord  is  nearly  circular  in  cross-section,  is  somewhat  enlarged 
in  the  regions  of  the  appendages,  tapers  toward  the  posterior 
end  and  is  divided  into  symmetrical  right  and  left  lobes  by  a 
dorsal  and  a  ventral  longitudinal  groove  (see  Fig.  171,  df.).  It 
possesses  a  central  canal  continuous  with  the  cavities  of  the 
brain.  The  outer  part  of  the  cord  (Fig.  171,  w.)  is  composed 
of  the  white  matter  (longitudinal  nerve  fibres)  and  the  interior 
portion,  of  gray  matter  (a  mixture  of  nerve-cells  and  fibres). 
This  is  somewhat  the  reverse  of  the  condition  found  in  the 
brain. 


CHORDATA. 


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ZOOLOGY. 


360.  Peripheral  Nervous  System  —  Spinal  Nerves.— 
Groups  of  nerve  fibres  spring  from  the  gray  matter  of  the 
cord  and  pass  to  the  organs  of  the  body.  These  nerves  arise 
in  pairs  —  one  pair  to  each  body  segment  —  and  pass  out  be- 


FIG.  171.  Diagram  of  a  cross-section  of  the  spinal  cord  through  the  roots  of  spinal 
nerves.  Drawn  by  Folsom.  c,  central  canal;  d.f.,  dorsal  fissure;  d.r.,  dorsal  root  of 
spinal  nerve  arising  from  the  dorsal  horn  of  the  gray  matter  (g) ;  gn.,  ganglion  on  the 
dorsal  root;  n,  spinal  nerve;  v.f.,  ventral  fissure;  i\r.,  ventral  root  of  the  spinal  nerve, 
arising  from  the  ventral  horn  of  the  gray  matter;  w.,  white  matter.  (The  dorsal  fissure 
in  the  diagram  is  broader  than  it  should  be.) 

Questions  on  the  figure. — What  is  the  structural  difference  between  the 
white  and  gray  matter  in  the  cord?  Describe  their  arrangement.  How 
are  the  two  halves  of  the  cord  united?  Which  are  sensory  and  which 
motor  roots?  What  structural  differences  do  you  notice  in  the  roots? 

tween  the  vertebrae.  Each  nerve  has  two  "  roots,"  a  dorsal 
and  a  ventral,  from  each  of  which  some  of  its  fibres  come 
(Fig.  171,  d.r.,  v.r.).  The  roots  differ  in  appearance  in  that 
the  dorsal  has  an  enlargement  (ganglion)  containing  nerve 
cells ;  the  ventral  has  none.  The  fibres  from  these  two  roots 
combine  to  form  the  nerve,  but  each  fibre  remains  independent 
throughout.  It  is  known  by  experiment  that  the  fibres  of  the 
dorsal  root  carry  impulses  toward  the  spinal  cord  ("  sensory  ") 
and  those  of  the  ventral  root  carry  impulses  from  the  cord 
("  motor").  In  certain  regions  the  nerves  springing  from 
successive  segments  of  the  body  may  have  numerous  inter- 
lacing fibres,  forming  what  is  known  as  a  plexus. 

361 .  Cranial  Nerves. — Those  nerves  arising  from  the  brain, 
that  is,  inside  the  cranium,  are  called  cranial  nerves.  There 
are  ten  to  twelve  pairs  of  these,  but  they  are  not  of  equal  mor- 


CHORDATA.  349 

phological  value,  nor  are  they  strictly  equivalent  to  the  spinal 
nerves.  Some  have  dorsal  and  ventral  roots,  but  a  much  larger 
number  have  only  one  group  of  roots,  either  dorsal  or  ventral. 
Some  are  purely  sensory  nerves,  some  are  motor  and  some  are 
mixed.  How  these  nerves  are  related  to  the  segments  of  which 
the  head  is  believed  to  be  composed  is  yet  an  unsettled  question. 

The  first  or  olfactory  arises  from  the  olfactory  lobe  of  the  fore-brain; 
its  fibres,  which  are  purely  sensory,  are  distributed  to  the  lining  of  the 
nose,  the  end  organ  of  smell. 

The  second  or  optic  nerve  arises  from  the  second  division  of  the  brain 
(thalaniencephalon),  consists  of  purely  sensory  fibres,  and  is  distributed  to 
the  retina  of  the  eye,  the  end  organ  of  vision. 

The  third,  fourth  and  sixth  pairs  are  purely  motor  and  are  distributed 
to  the  muscles  of  the  eye.  The  third  and  fourth  arise  from  the  third 
division  of  the  brain  (mesencephalon).  The  sixth  nerve  arises  from  the 
medulla,  as  do  the  following: 

The  fifth  (trigeminal')  comes  from  the  anterior  portion  of  the  medulla 
(hind-brain)  and,  like  the  spinal  nerves,  has  both  dorsal  and  ventral  roots. 
It  is  largely  sensory,  supplying  the  skin  of  the  face,  mouth  and  tongue. 
Motor  fibres  pass  to  the  muscles  of  the  jaw. 

The  seventh  (facial)  is  largely  motor,  is  distributed  chiefly  to  the 
muscles  of  the  face  and  controls  facial  expression. 

The  eighth  or  auditory  is  sensory,  and  is  distributed  to  the  inner  ear, 
the  end  organ  of  hearing  and  of  equilibration. 

The  ninth  or  glossopharyngeal  is  a  mixed  nerve  and  is,  distributed  to 
the  muscles  and  mucous  membrane  of  the  pharynx  and  to  the  tongue. 

The  tenth  or  vagus  arises  by  numerous  roots,  has  both  motor  and  sen- 
sory fibres,  and  is  the  most  widely  distributed  nerve  in  the  body.  Its  fibres 
pass  to  the  posterior  visceral  arches,  lungs,  heart,  stomach  and  intestines. 

We  find  the  cranial  nerves  and  their  nerve  endings  concerned  chiefly 
with  the  higher  senses,  the  muscles  of  expression,  and  the  sensations  and 
activities  involved  in  the  fundamental  processes  of  nutrition. 

362.  The    Sympathetic    System   which   is    always   distributed   to   the 
visceral  organs  is  made  up  of  a  series  of  connected  ganglia  in  the  dorsal 
part  of  the  body  cavity.     This  system  is  in  connection  at  various  places 
with  the  central  nervous  system.     It  serves  to  connect  the  internal  organs 
more  intimately,  and  is  reflex  in  its  action. 

363.  Organs  of  Special  Sense. — The  sense  organs  repre- 
sent specialized  terminations  of  the  nerve  fibres,  or  special 
epithelial  cells  which  have  become  associated  with  such  fibres 
(Fig.  41).     From  the  very  nature  of  the  case  they  must  be 
external.     In  the  case  of  higher  animals,  the  more  compli- 


35°  ZOOLOGY. 

cated  sense-organs  are  removed  from  the  surface  and  are  much 
modified,  but  the  essential  sensory  portion  is  similar  in  all, 
and  they  retain  some  suitable  connection  with  the  outside.  It 
is  usually  these  accessory  connecting  structures  which  render 
the  sense  organ  so  complicated. 

364.  The  Skin  Senses. — Scattered  over  the  body  of  many 
forms  of  animals  are  single  cells,  or  groups  of  cells,  or  free 
nerve  endings,  which  are  for  the  reception  of  contact  and  tem- 
perature stimuli.     These  are  not  equally  numerous  or  well 
developed  in  all  parts  of  the  body.     They  are  often  especially 
developed   in   connection  with  hairs.      In  the  lower  aquatic 
vertebrates,  especially  the  fishes,  groups  of  such  sensory  cells 
occur  in  pits  or  longitudinal  grooves  along  the  sides.     These 
are  called  the  organs  of  the  lateral  line.    Their  exact  function 
is  still  in  some  doubt.     They  may  possibly  assist  in  the  de- 
termination  of   the   chemical   condition  of   the   water   or   in 
determining  the  equilibrium  of  the  animal  in  the  water. 

365.  The    Chemical    Senses— Taste    and    Smell.— The 
chemical  senses  involve  close  contact  and  a  chemical  union 
between  the  substance  to  be  perceived  and  the  organ  itself. 
For  that  reason  the  substance  must  be  capable  of  solution  in 
the  fluids   that  moisten   the  surfaces.     In   vertebrates   these 
organs  are  located  at  the  anterior  end  of  the  body  and  usually 
within  special  pits  or  cavities.     The  taste  organs  are  in  the 
mouth,  especially  on  the  tongue  and  soft  palate.    In  some  ani- 
mals the  sense  is  poorly  developed.     The  end  organs  of  the 
sense  of  smell  are  located  in  pits,  anterior  or  dorsal  to  the 
mouth,  lined  with  folds  of  the  mucous  epithelium.     In  most 
fishes  these  pits  are  not  connected  with  the  pharynx,  but  in  all 
air-breathing  forms  there  is  such  connection,  and  the  nostrils 
constitute  the  normal  inlet  for  air  to  enter  the  lungs.     The 
sense  of  smell  is  much  more  developed  in  the  air-breathing 
vertebrates,  if  indeed  it  can  be  said  to  exist  at  all  in  the  aquatic 
animals. 

366.  The  Ear. — The  vertebrates  have  a  single  pair  of  ears, 
and   these  are   located   at  the   side  of   the  head  behind   the 


CHORDATA.  351 

eyes.  The  essential  sensory  portion  of  the  ear  (internal  ear) 
arises  as  an  inpocketing  of  ectoderm,  and  consists  of  a  closed, 
fluid-filled  membranous  sac  which  is  surrounded  by  mesoder- 
mal  structures — often  solid  bone.  Ordinarily  this  sac  is  con- 
stricted, being  thus  partially  separated  into  two  irregular  cham- 
bers, the  dorsal  (utriculus)  and  the  ventral  (sacculus).  From 
the  former  arise  three  semicircular  canals  which  are  supplied 
with  sensory  hair-cells  in  the  epithelial  lining  and  are  looked 
upon  as  being  an  organ  to  assist  in  detecting  direction  of 
motion  and  maintaining  balance  or  equilibrium.  From  the 
sacculus  arises  an  outgrowth,  the  cochlea,  which  in  higher 
forms  is  well  developed.  It  becomes  spiral,  and  is  well  sup- 
plied with  sensory  cells.  It  is  regarded  as  the  chief  auditory 
organ  in  those  forms  possessing  it.  This  membranous  sac  or 
labyrinth  is  completely  surrounded  by  cartilage  or  bone  in 
fishes,  and  lies  toward  the  middle  line  from  the  spiracle.  There 
is  no  external  ear.  In  forms  above  the  fishes  a  membrane 
(tympanic  membrane)  stops  up  the  spiracle  and  incloses  what 
is  known  as  the  middle  ear,  which  still  communicates  with  the 
mouth  by  the  Eustachian  canal.  A  bridge  of  minute  bones  is 
also  formed  from  the  tympanic  membrane  across  the  middle 
ear  whereby  the  external  vibrations  can  be  communicated  to 
the  internal  ear.  In  addition  to  this,  particularly  among  the 
mammals,  is  found  an  external  tube  leading  to  the  tympanic 
membrane.  Expanded  folds  of  skin  supported  by  cartilage 
form  a  funnel  to  catch  the  waves.  The  tube  (external  audi- 
tory meatus}  and  the  funnel  or  pinna  constitute  the  external 
ear. 

367.  The  Eye. — The  eyes  of  vertebrates  are  a  single  pair 
of  organs  lying  imbedded  in  an  orbit  of  cartilage  or  bone, 
within  which  they  have  considerable  freedom  of  motion. 
Six  muscles,  four  straight  (rectus)  and  two  oblique  serve  to 
move  the  eyeball.  These  muscles  receive  the  third,  fourth, 
and  sixth  of  the  cranial  nerves.  In  the  higher  forms  muscular 
folds  of  the  skin  serve  to  protect  the  eye  in  front.  The  upper 
and  lower  lids  act  vertically,  but  the  third  (nictitating  mem- 


352 


ZOOLOGY. 


brane)  works  from  the  inner  angle  of  the  eye  outward.  Some- 
times all  three  lids  are  present  together.  In  the  lower  groups 
the  lids  are  wanting. 

The  essential  part  of  the  eye  is  the  sensory  expansion  of  the 
optic  nerve — the  retina — which  occupies  the  innermost  posi- 
tion, bounding  the  posterior  portion  of  the  cavity  of  the  eye- 
ball. This  is  a  very  complicated  layer,  but  a  general  idea  of 
it  can  be  obtained  from  the  diagram  (Fig.  173).  The  layer 
of  rods  and  cones,  in  close  connection  with  a  layer  of  pigment, 

FIG.  172. 


FIG.  172.  Diagrammatic  horizontal  section  through  the  right  eye  of  Man.  The  line 
a  p  is  the  axis  of  vision.  The  optic  nerve  leaves  the  eye  on  the  median  side  of  this 
line,  a.c.,  central  artery;  o./t.,  aqueous  humor;  b.,  blind  spot,  the  entrance  of  the 
optic  nerve;  c,  conjunctiva;  ch.,  choroid  layer  of  the  eye-ball;  c.l.,  crystalline  lens; 
c.m.c.,  circular  fibres  of  the  ciliary  muscles;  c.m.r.,  radial  fibres  of  the  ciliary  muscles; 
co.,  cornea,  the  transparent  portion  of  the  sclerotic;  c.p.,  ciliary  process;  c.s.,  canal  of 
Schlemm,  a  lymphatic  vessel;  fo.,  fovea  centralis,  the  point  of  clearest  vision;  o.n., 
optic  nerve;  o.s.,  ora  serrata,  the  anterior  wavy  margin  of  the  visual  portion  of  the 
retina;  r,  the  retinal  layer;  sc.,  sclerotic  layer;  sh.,  sheath  of  the  optic  nerve;  v.h., 
vitreous  humor. 

Questions  on  the  figure. — Which  is  the  essential  sensory  portion  of  the 
eye?  Which  parts  are  concerned  in  bringing  the  rays  of  light  to  a  focus? 
How  many  refractive  surfaces  are  present?  How  many  refractive  media? 
Which  portions  of  the  eye  are  primarily  protective  and  supportive?  What 
is  the  function  of  the  various  parts  of  the  choroid  layer?  In  what  way  js 
an  image  formed  on  the  retina,  of  objects  in  front  of  the  lens? 


CHORDATA.  353 

is  the  sensitive  layer.  Light  falling  directly  on  this  from  all 
directions  would  produce  no  image,  just  as  the  photographic 
plate  exposed  outside  the  camera  would  present  a  general  blur. 
We  find  then  the  necessity  of  the  same  optical  devices  as  are 
found  in  the  camera:  (i)  a  sensitive  surface,  the  retina;  (2) 
a  box  for  support  and  for  keeping  out  the  light  except  from 

FIG.  173. 


FIG.  173.  Diagram  showing  some  of  the  retinal  elements  in  their  relations  to  each 
other.  Layer  i  is  directed  toward  the  interior  of  the  eye  and  consists  of  nerve  fibres 
(f)  which  finally  enter  the  optic  nerve  at  the  blind  spot;  2,  the  ganglion-cell  layer, 
made  up  of  the  nerve  cells  of  which  the  fibres  are  a  part;  3,  the  inner  "  molecular 
layer"  made  up  of  the  fine,  much-branched  nerve  fibrils  from  2  and  4;  4,  the  inner 
nuclear  layer,  containing  numerous  ganglion  cells  (g)  ;  5,  the  outer  "  molecular  layer," 
similar  in  structure  to  3 ;  6,  outer  nuclear  layer,  containing  the  nuclei  of  the  rod  and 
cone  cells  (r.c.  and  c.c.)  ;  7,  the  layer  of  rods  and  cones  (r,  c).  This  is  the  nervous 
epithelium,  or  the  nerve-endings  of  vision.  The  rods  and  cones  are  partly  imbedded 
in  a  pigment  epithelium  (8).  It  must  be  remembered  that  hundreds  of  elements  are 
omitted  where  one  is  shown  in  the  figure. 

Questions  on  the  figure. — Which  portion  of  the  retina  does  the  light 
first  strike  on  entering  the  eye?  To  what  point  must  it  penetrate  to  arouse 
nervous  activity?  Over  what  route  must  a  nervous  impulse  pass  to  reach 
the  brain  from  the  point  of  stimulation  (rods  and  cones)  ?  Compare  with 
similar  figures  in  other  texts. 
24 


354  ZOOLOGY. 

one  direction,  the  opaque  layers  of  the  ball  of  the  eye;  (3) 
an  aperture  for  the  passage  of  the  light  into  the  interior  of 
the  box,  the  pupil  and  the  transparent  cornea  overlying  it ;  and 
especially  (4)  a  lens  or  series  of  refracting  surfaces  which 
cause  all  the  rays  of  light  coming  to  the  eye  from  each  external 
point  to  be  brought  together  again  beyond  the  lens  at  a  corre- 
sponding point  on  the  sensitive  surface.  The  elementary  rela- 
tions of  these  parts  as  found  in  the  eyes  of  vertebrates  may  be 
gathered  from  a  study  of  Fig.  172. 

Accommodation  of  the  eye  to  objects  at  different  distances 
is  effected  by  means  of  changes  in  the  shape  of  the  lens  through 
the  action  of  appropriate  muscles. 

368.  Library  Exercise. — What  portions  of  the  vertebrate 
eye  are  derived  directly  from  the  ectoderm?   Which  from  the 
brain  (i.  e.,  indirectly  from  ectoderm)  ?    Which  from  meso- 
derm?   (See  Fig.  43.) 

What  variation  occurs  among  vertebrates  as  to  the  condi- 
tion of  the  bones  in  the  middle  ear?  Whence  are  they  con- 
sidered to  be  derived?  What  variation  in  the  cochlea?  Study 
from  figures  the  structure  of  the  cochlea. 

369.  Classification. — The  principal  divisions  of  the  sub- 
phylum  vertebrata  are : 

Class  I.  Pisces  (e.  g.}  sharks,  lung-fishes,  bony  fishes). 
Page  355. 

Class  II.  Amphibia  (frogs,  toads,  salamanders,  etc.). 
Page  372. 

Class  III.  Reptilia  (crocodiles,  lizards,  snakes,  turtles). 
Page  380. 

Class  IV.  Aves  (birds).     Page  394. 

Class  V.  Mammalia  (mammals).     Page  427. 


CHAPTER    XX. 
CLASS  L— PISCES. 

370.  The    class    of    fishes,    representatives    of    which    are 
familiar  to  all,  is  important  not  only  from  the  point  of  view 
of  its  specialized  present-day  representatives  but  from  the  fact 
that  it  was  the  first  successful  vertebrate  group  of  geological 
times.     It  represents  the  primitive  aquatic  habit  from  which 
the   land-inhabiting  types   of   vertebrates   must  have   arisen, 
and  in  it  we  find  the  fundamental  plan  of  structure  which  has 
been  modified  in  the  higher  forms   (as  the  Amphibians)   in 
adaptation  to  their  present  mode  of  life.     It  must  of  course 
be  borne  in  mind  that  the  types  of  fishes  which  are  supposed 
to  be  the  ancestors  of  the  air-breathing  vertebrates  were  much 
less  specialized  in  structure  than  the  present  members  of  the 
class.    There  are,  however,  even  now  some  of  the  fishes  which 
have  changed  less  than  the  majority,  from  the  primitive  condi- 
ton. 

371.  General  Characteristics. — 

1.  Fishes   are   aquatic  vertebrates   having  gills   functional 
throughout  life.     These  consist  of  filaments  or  sheets,  con- 
taining blood-vessels  and  attached  to  bony  or  cartilaginous 
arches  in  the  region  of  the  pharynx. 

2.  Paired  appendages    (pectoral  and  pelvic)   are  normally 
fin-like, — not  having  a  median  jointed  axis  as  in  the  limbs  of 
the  higher  vertebrates.     The  medial  fins  are  dorsal,  ventral, 
and  caudal.    The  last  is  the  chief  organ  of  locomotion. 

3.  There  is  usually  a  dermal  skeleton  consisting  of  scales, 
covered  with  epidermis.     The  latter  may  deposit  enamel  on 
the  dermal  core  of  the  scale. 

4.  There  is  a  two-chambered  heart  through  which  the  sys- 
temic (impure)  blood  flows. 

•355 


35 6  ZOOLOGY. 

5.  Vertebral  column  either  cartilaginous  or  bony;  the  verte- 
brae biconcave. 

6.  There  is  no  true   (allantoic,  see  Fig.  205,  a/.)   urinary 
bladder. 

7.  A  longitudinal   line  of  sense  organs    ("  organs   of  the 
lateral  line  ")  on  each  side  of  the  body. 

8.  Nasal  pit  does  not    (usually)    communicate  posteriorly 
with  the  mouth. 

9.  Fertilization   and   development  usually  take  place   out- 
side the  body. 

372.  Form. — Fishes  usually  have  a  body  somewhat  flattened 
from  side  to  side,  though  it  may  be  quite  cylindrical  or  flat- 
tened dorso-ventrally,  and  gradually  tapering  toward  either 
end.     This  is  readily  seen  to  be  a  form  well  suited  to  motion 
through  water.     The  head  end,  while  not  so  specialized  as  in 
the  higher  forms,  is  much  more  cephalized  than  in  the  Proto- 
vertebrata.     The   mouth   with   its   modifications   of   movable 
jaws,  teeth,  etc.,  the  respiratory  arches,  the  sense  organs  of 
sight,  hearing,  taste,  and  possibly  smell,  the  brain  and  brain 
case  all  enter  into  this  cephalization. 

There  is  no  neck,  i.  e.}  the  head  is  not  movable  with  refer- 
ence to  the  body.  The  length  of  the  body  varies  very  greatly 
as  does  the  number  of  metameres  embraced.  The  body  may 
be  distinguished  as  pre-caudal  and  caudal. 

373.  Appendages. — Fishes  possess  two  classes  of  append- 
ages— paired,   or  lateral,  and   median.     The  paired   fins   are 
four  in  number  and  are  considered  to  be  homologous  with 
the  pectoral  and  pelvic  appendages  of  the  higher  vertebrates. 
They  differ  much  as  to  their  position,  especially  the  posterior 
pair,  as  may  be  seen  by  a  comparison  of  the  figures  in  this 
chapter.     In  its  typical  condition  the  appendage  consists  of 
girdle  and  the  fin  proper.    The  skeletal  supports  may  be  either 
cartilaginous  or  bony.     In  the  lung-fishes  there  is  a  central 
axis  (Fig.  174)  through  the  fin,  instead  of  the  usual  radiate 
arrangement  (Fig.  175,  f.r.)  of  the  fin-rays.    The  legs  of  higher 


PISCES. 


357 


vertebrates  are  supposed  to  have  been  derived  from  this  type 
(the  archipterygium).  The  median  fins  consist  of  one  or  more 
dorsal  portions  which  vary  in  extent,  a  caudal  portion,  and  a 

FIG.  174. 


FIG.   174.     Diagram  of   the   anterior    fin    (archipterygium)    of    Ceratodus. 

Questions  on  the  figure. — What  are  the  chief  points  of  contrast 
between  this  fin  and  that  of  Teleosts,  figured  in  Fig.  175?  How  does 
Gegenbauer  consider  that  this  might  give  rise  to  the  simpler  type  of 
Vertebrate  legs? 

FIG.  175. 


FIG.  175.  Pectoral  girdle  and  fin  of  a  Teleost.  br.o.,  branchial  ossicles;  c,  coracoid; 
cl.,  clavicle;  f.r.,  fin  rays;  p.cl.,  post  clavicle;  p.t.,  post  temporal  which  unites  with 
skull;  sc.,  scapula;  s.cl.,  supra-clavicle.  By  Folsom. 

Questions  on  the  figure. — Which  girdle  is  this,  right  or  left?  Do 
authors  regard  any  of  these  bones  as  homologous  with  similarly  named 
bones  in  the  higher  Vertebrates? 


358 


ZOOLOGY. 


ventral,  near  the  anus.  These  may  represent  remnants  of  a 
continuous  median  fin  such  as  is  seen  mAmphioxus  (Fig.  152). 
They  are  supported  by  fin-rays  in  the  dermal  fold,  which 
are  in  turn  supported  by  spines  imbedded  in  the  muscles.  The 
form  of  the  caudal  fin,  which  is  much  used  in  locomotion, 
differs  widely.  These  differences,  correlated  with  modifica- 

FIG.  176. 


FIG.  176.  Diagrams  of  some  principal  forms  of  tails  in  fishes.  A,  protocercal  fin 
(as  in  Polypterus) ;  B,  heterocercal  (as  in  Sharks)  ;  C,  homocercal  (as  in  most  Teleosts) ; 
D,  homocercal  (as  in  Amia).  By  Folsom. 

Questions  on  the  figure. — What  is  the  essential  difference  between  the 
symmetry  of  D  and  of  A?  What  conceivable  advantage  has  the  sym- 
metrical over  the  unsymmetrical  type?  Are  the  heterocercal  types  success- 
ful swimmers? 

tions  of  the  end  of  the  vertebral  column,  have  considerable 
importance  in  subdividing  the  class.  The  following  types 
may  be  noted: 

1.  The  vertebral  column  passes  straight  to  the  end  of  the 
tail  and  the  fin-rays  are  disposed  symmetrically  with  regard 
thereto  (protocercal}  ;  found  in  lung-fishes  and  some  primitive 
extinct  forms  (Fig.  176,  A). 

2.  The  vertebral  column  is  bent  dorsad,  and  a  small  fin  lobe 
develops  from  its  ventral  side.    The  tail,  though  two-pronged, 
is  not  symmetrical  (heterocercal).    Found  in  sharks  and  many 
ganoids  (Fig.  176,  B). 


PISCES. 


359 


3.  The  vertebral  column  may  become  still  more  bent  and 
reduced;  the  ventral  lobe  develops  until  the  whole  structure 
appears  symmetrical  again  (homocercal).  Found  in  bony- 
fishes  (Fig.  176,  C,  D). 

FIG.  177. 


FIG.  177.  Skull  of  Cod  (Gadus  morrhua).  From  Nicholson,  after  Owen,  b, 
branchiostegal  rays  borne  on  c.h.,  the  ceratohyal  bone;  d,  dentary  portion  of  the 
mandible;  f,  frontal;  h.m.,  hyomandibular;  i.o.,  interoperculum;  I,  lachrymal;  m, 
maxilla;  n,  nasal;  o,  operculum;  p.m.,  premaxilla;  p.o.,  preopercuhim;  p.s.,  parasphe- 
noid;  q,  quadrate;  s.o.,  sub-operculum ;  s.oc.,  supra-occipital. 

Questions  on  the  figure. — What  is  the  operculum?  How  many  bones 
are  associated  to  form  it?  Which  bones  are  figured  as  bearing  teeth? 
Which  of  these  bones  belong  to  the  cranium  proper?  What  is  the  differ- 
ence between  cranium  and  skull?  What  do  authors  believe  to  be  the 
origin  and  homology  of  the  chief  facial  bones? 

374.  Covering. — Most  fishes  are  more  or  less  covered  by 
scales  or  scutes  of  bony  matter  developed  in  the  dermis  and 
lying  between  the  dermis  and  epidermis.  The  scales  often 
receive  a  layer  of  enamel  from  the  epidermis.  In  form  they 
may  be  cycloid  (round,  with  smooth  margin),  ctenoid  (toothed 
margin),  placoid  (plate-like  bodies  often  bearing  points 
covered  with  enamel),  and  ganoid  (thick  rhomboid  scales 
covered  over  with  enamel,  and  often  closely  articulated  into 
a  coat-of -armor).  A  good  many  species  of  fishes  are  destitute 


360  ZOOLOGY. 

of  scales  altogether,  the  skin  of  such  often  being  supplied  with 
numerous  mucous  glands.  In  many  extinct  forms  the  external 
covering  was  made  up  of  large  plates  fused  into  a  dense  armor. 

375.  The   Skull. — The   skull   in   fishes   is   especially  note- 
worthy for  the  looseness  of  the  connection  between  the  facial 
bones  (i.  e.,  the  visceral  or  branchial  arches)  and  the  cranium. 
They  are  readily  separated   from  the  cranium.     The  lower 
jaw  is  not  articulated  directly  with  the  brain-case  but  with  the 
upper  jaw  (see  Fig.  177,  q). 

376.  Locomotion. — Fishes   are  aquatic  and  are  complete 
masters  of  their  medium.     The  density  of  water  as  compared 
with  air  makes  the  matter  gf  support  in  the  medium  much 
easier  for  the  fish  than  for  the  bird.     The  denser  medium  is 
however  more  difficult  to  penetrate.     The  specific  gravity  of 
the  fish  as  a  whole  does  not  differ  widely  from  that  of  water, 
although  it  varies  within  narrow  limits.     Three  problems  are 
thus  presented  to  the  fish  for  solution : 

1.  The  Regulation  of  Specific  Gravity. — This  is  effected  in 
part  at  least  by  the  air  bladder.     The  body  muscles  may  bring 
about  the  compression  of  the  contained  gas  and  thus  decrease 
the  size  without  change  of  weight. 

2.  Propulsion. — The  chief  organ  of  propulsion  is  the  caudal 
fin,  acted  on  by  the  powerful  lateral  muscles  of  the  body.    The 
resultant  of  the  alternate  strokes  against  the  water  is  forward 
motion.    This  may  be  supplemented  by  the  action  of  the  paired 
fins. 

3.  Steering. — This  is  accomplished  in  part  by  the  changes 
in  specific  gravity  and  the  regulation  of  the  stroke  of  the  tail, 
and  in  part  by  the  action  of  the  paired  fins.    The  semi-circular 
canals  probably  assist  the  animal  in  appreciating  changes  in 
its  position, — its  orientation,   thus  enabling  it  to  choose  its 
direction. 

377.  Supplementary  Exercise  for  the  Library. — What  is  the  structure 
and  position  of  the  "  swim-"  or  air-bladder  in  fishes  ?     With  what  organ 
is  it  related?    Does  it  communicate  with  the  outside?    Are  there  any  evi- 
dences that  it  is  of  value  as  n   respiratory  organ  in   any  of  the   fishes? 


PISCES. 


361 


C.v.L 


FIG.  178.  Diagram  of  the  principal  vessels  in  the  circulation  of  a  fish, — -ventral 
view,  a,  aorta;  an.,  auricle;  br.a.,  afferent  branchial  arteries;  br.e.,  efferent  branchial 
arteries;  bu.,  bulbus  (or  conus)  arteriosus;  c,  carotid;  c.a.,  caudal  artery;  c.r.r.,  right 
cardinal  vein;  c.v.l.,  left  cardinal  vein;  d.a.,  dorsal  artery;  d.c.,  ductus  Cuvieri;  g, 
gills;  h.v.,  hepatic  vein;  h.p.,  hepatic  portal  vein;  i,  intestine;  /,  jugular  vein;  k, 
kidney;  /,  liver;  r.p.,  renal  portal  vein;  s.v.,  sinus  venosus;  v,  ventricle. 

Questions  on  the  figure. — Follow  the  general  course  of  the  circulation, 
noting  changes  in  the  character  of  the  blood  in  various  capillary  regions. 
What  is  the  extent  of  the  hepatic  portal  system  ?  Of  the  renal  portal  ? 
Where  is  the  purest  blood  in  the  body?  Reason  for  your  answer?  What 
do  you  mean  by  "impure"  blood?  Where  are  the  chief  impurities  re- 
moved from  the  circulation  ? 


362  ZOOLOGY. 

Can  yon  conceive  any  use  it  might  be  in  steering,  for  the  purpose  of  rising 
or  sinking  in  the  water?  What  would  be  the  effect  of  compressing  the 
air-bladder  at  one  end  more  than  at  the  other? 

378.  The  Circulation. — Little  needs  be  said  here  in  addition 
to  what  has  been  said  in  the  general  discussion  of  the  verte- 
brate circulation  (see  Fig.  164-169).  The  heart  is  two-cham- 
bered. The  auricle  receives  the  venous  blood  from  the  system ; 
it  is  passed  to  the  ventricle  through  a  valve  which  forbids  its 
passage  in  the  reverse  direction.  From  the  ventricle  the  blood 
passes  through  a  valvular  region  into  the  ventral  aorta,  which 
carries  it,  by  a  series  of  right  and  left  branches,  to  the  gills. 
Here  aeration  takes  place,  the  pure  blood  being  gathered  from 
the  gills  by  a  series  of  efferent  branches  which  combine  (ex- 
cept some  anterior  branches  which  go  to  the  head)  to  form  a 
dorsal  aorta.  The  dorsal  aorta  gives  off  branches  to  the  body 
wall,  to  the  paired  appendages,  to  the  liver,  digestive  tract  and 
kidneys, — continuing  into  the  tail  where  it  breaks  up  in  the 
muscles.  The  impure  blood  from  the  capillaries  of  the  tail  is 
brought  back  to  the  kidneys  by  the  renal  portal  vein,  where  it 
again  passes  through  capillaries ;  here  the  blood  is  purified  of 
its  urea  and  similar  impurities.  The  blood  supplied  direct  to 
the  kidneys  from  the  aorta  and  that  of  the  renal-portal  circu- 
lation is  returned  to  the  heart  by  way  of  right  and  left  (cardi- 
nal) veins  which  join  corresponding  right  and  left  veins 
(jugular)  from  the  head  to  form  the  veins  (ductus  Cuvieri) 
which  empty  into  the  auricle.  The  blood  which  was  distributed 
to  the  stomach  and  intestines  is  gathered  into  a  vessel  (hepatic 
portal  vein)  which  carries  it  to  the  liver,  together  with  much 
of  the  food  absorbed  from  the  intestines.  The  hepatic  portal 
vein  here  breaks  up  into  capillaries.  The  blood  from  the  liver 
and  from  the  appendages  unites  with  that  carried  by  the 
ductus  Cuvieri  before  it  reaches  the  heart.  The  student  should 
carefully  follow  out  the  course  of  the  circulation  in  the  accom- 
panying diagrams  (Figs.  178,  179).  Variations  from  this 
typical  condition  are  numerous,  accounts  of  which  must  be 
sought  in  more  extended  texts. 


FIG.   179.     Diagram  of  the  principal  vessels  in  the  circulation  of  a  Fish, — lateral  view. 
Lettering  as  in  the  preceding  figure.     Adapted  from  Parker  and  Haswell. 

Questions  on  the  figure. — Compare  the  two  views  (Figs.  178  and  179) 
and  identify  the  parts  common  to  both,  tracing  the  course  of  the  circula- 
tion in  the  various  vessels.  Put  arrows  in  the  figure  to  show  your  idea 
of  the  course  of  the  blood. 

379.  Library  Exercise. — Find  description  and  figures  in  the  reference 
zoologies  locating  the  unfamiliar  structures  in  the  table  on  page  340  and 
testing  the  statements  found  there. 

380.  Habits  and  Food. — Fishes  occur  abundantly  both  in 
fresh  and  salt  water.     Usually  the  whole  life  is  spent  under 
the  same  general  conditions.     The  salmon  and  shad,  however, 
are  bred  and  partly  develop  in  fresh  water  and  later  pass  out 
to  the  sea.     These  forms  return,  often  with  remarkable  pre- 
cision, to  the  place  of  their  own  hatching  to  deposit  their  eggs. 
Use  is  made  of  this  habit  in  the  capture  of  them  for  com- 
mercial purposes.     Unless  some  means  are  found  for  limiting 
the  destruction  of  the  adults  during  the  breeding  period,  some 
of  our  most  valuable  food  fishes  are  in  danger  of  extermina- 
tion.    Others,  as  the  eels,  may  generate  in  the  sea  and  spend 
a  part  or  all  their  adult  life  in  fresh  water.     Some  burrow  in 
the  muddy  bottoms,  as  the  eel,  cat-fish,  mud-fish;  others  lie 
on  the  bottom,  as  the  flat-fish ;  many  quaint  forms  frequent  the 
depths  of  the  sea.     The  most  are  active  swimmers  in  open 
water  near  or  away  from  the  shore.     Many  forms  (herring, 
shad,  salmon,  etc.)  are  distinctly  gregarious,  moving  in  great 
shoals  especially  at  spawning  time  or  when  in  search  of  food. 
This  fact  and  the  knowledge  of  places  and  times  are  matters 


ZOOLOGY. 


of  much  moment  to  the  fishermen.  The  food  of  fishes  is  very 
diverse.  Some  forms  are  actively  carnivorous,  preying  on 
animals  as  large  or  larger  than  themselves  (sharks)  ;  others, 
and  these  are  the  most  numerous  class,  depend  upon  small 
animals  such  as  the  young  of  their  own  or  other  species  of  fish, 
on  Crustacea,  insects  and  worms.  The  microscopic  animals  and 
plants  occurring  in  immense  numbers  in  the  water  are  im- 
portant items  in  the  food  of  fishes.  Some  fishes  are  scaven- 
gers, living  largely  upon  the  dead  materials  found  in  the  water. 
Fishes  differ  much  in  their  energy,  courage,  and  resistance  to 
attack.  Those  possessing  these  qualities  in  high  degree  are 
denominated  "  game  "  fish  and  are  prized  for  the  difficulty 
involved  in  their  capture.  The  family  of  the  trout  and  salmon 
includes  several  such  species. 

381.  Economic  Value. — From  primitive  times  fish  has  been 
one  of  the  important  human  foods.  Probably  a  larger  per- 
centage of  the  well-known  species  of  fishes  are  regarded  as 

FIG.  1 80. 


FIG.   180.     Atlantic    Salmon     (Salmo    salar).     From    the    "  Manual    of    Fish    Culture," 

U.  S.  F.  C. 

Questions  on  the  figure. — What  are  the  names  of  the  various  fins 
shown  in  the  figure?  What  is  the  dotted  line  along  the  side  of  the  fish? 
What  type  of  tail  has  this  fish? 

edible  than  of  any  other  animal  group.  Their  rate  of  multi- 
plying and  their  occurrence  in  schools  at  available  points  are 
quite  as  important  factors  as  the  delicacy  of  the  flesh  in  de- 


PISCES.  365 

termining  the  food  value  of  a  species.  The  improved  devices 
for  capturing  fish,  the  development  of  methods  of  preserving 
them  by  drying  and  by  canning,  and  the  increased  price  of 
other  food  substances  for  which  fish  may  be  substituted  have 
all  conspired  to  increase  the  destruction  of  the  more  important 
edible  fish  both  in  the  fresh  and  salt  waters.  In  recognition 
of  this,  most  nations  have  appointed  commissions  for  the  study 
of  problems  connected  with  the  fisheries  and  for  the  better 
regulation  of  the  same.  The  United  States  Fish  Commission 

FIG.  181. 


FIG.   18 1.     Brook    Trout    (Salvelimus    fontinalis).     From    "  Manual    of    Fish    Culture," 

U.  S.   F.  C. 

in  conjunction  with  similar  state  boards,  has  done  an  im- 
measurable amount  of  good  especially  in  the  following  par- 
ticulars : 

1.  In  taking  the  spawn  of  our  most  important  food  fishes 
and  caring  for  it  artificially  during  the  period  of  early  develop- 
ment when  the  young  animals  are  in  the  greatest  danger  of 
destruction.     Such  fish  hatcheries  are  scattered  all  over  the 
Union  and  many  of  our  fresh  waters  are  being  restocked  with 
species  believed  to  be  hardy  and  suitable  for  food. 

2.  By  determining  the  foods  preferred  by  special  fish  and 
artificially  encouraging  its  abundance. 


366  ZOOLOGY. 

3.  By  studying  the  habits  of  the  fishes  and  by  regulation  of 
the  time,  place  and  manner  of  catching. 

382.  Supplementary  Exercises  for  the  Library. 

1.  Make    a    report   concerning   the   principal    food    fishes    used    by   the 
people  of  the  United  States :  their  habits  and  geographical  range,  the  mode 
of  their  capture  and  putting  on  the  market. 

2.  Make  a  study  of  the  methods  of  capturing  fish  from  primitive  time 
to  the  present  and  show  how  the  methods  have  been  adapted  to  the  habits 
of  the  fish. 

3.  A  study  of  the  history  and  work  of  the  United  States  Fish  Com- 
mission as  shown  in  the  annual  reports  and  bulletins.     Its  economic  side. 
Its  scientific  side. 

383.  Reproduction    and    Development. — The    sexes    are 
separate.    The  sexual  elements  are  produced  in  great  numbers. 
The  ova  (spawn)  are  usually  deposited  in  the  water,  in  shal- 
lows on  the  open  bottom,  under  rocks,  or  in  places  specially  pro- 
vided for  them  by  the  parents.    The  sperm  ( milt)  is  poured  over 
these  by  the  male,  and  the  fertilization  and  later  development 
take  place  in  the  water  with  little  or  no  care  on  the  part  of  the 
parents.     Great  loss  of  life  occurs  among  the  young  from  the 
voracious  habits  of  other  species  and  sometimes  of  the  parents 
themselves.     It  is  not  difficult  to  believe  that  the  enormous 
number  of  eggs  produced  by  the  female  is  ah  adaptation  to 
meet  this  risk  of  mortality  among  the  young.     In  some  cases 
(most  sharks  and  a  few  bony-fishes)  the  eggs  are  fertilized 
and  the  young  hatched  within  the  body  of  the  mother.     Only 
a  few  young  are  produced  in  such  forms. 

The  eggs  of  fishes  are  usually  well  supplied  with  yolk,  seg- 

FIG.  182. 


FIG.   182.     The  Smelt   (Osmerus  dentax).     Bull.  U.   S.  Fish  Commission. 


PISCES.  367 

mentation  being  partial  (discoidal).  The  unsegmented  por- 
tion comes  to  be  surrounded  by  a  yolk  sac  and  furnishes  nour- 
ishment for  the  early  stages  of  development. 

384.  Special  Adaptations. — In  addition  to  those  already 
mentioned  the  group  of  fishes  shows  many  adaptations  to 
special  modes  of  life. 

Color. — Most  fishes  show  color  as  the  result  of  pigment 
buried  in  the  cells  of  the  skin,  or  of  delicate  markings  on  the 
scales.  In  general,  the  tone  of  color  accords  with  the  environ- 
ment. This  becomes  very  striking  in  some  of  the  less  active 
forms,  as  the  flounders,  in  which  the  colors  may  change  more 
or  less  rapidly  to  accord  with  the  bottom  on  which  they  lie. 
It  seems  probable  that  some  degree  of  protection  from  enemies 
may  thus  be  gained,  which  would  be  of  distinct  value  to  the 
species.  Some  deep-sea  forms  are  phosphorescent.  This  is 
probably  of  considerable  importance,  as  no  sunlight  penetrates 
to  that  depth. 

Electrical  Organs. — In  several  groups  of  fishes  (rays,  eels, 
etc.)  certain  muscular  tracts  have  become  so  modified  that 
under  nervous  stimulus  instead  of  producing  motion  by  con- 
traction they  form  and  accumulate  electrical  energy  which  may 
be  discharged  at  the  will  of  the  animal.  This  power  certainly 

FIG.  183. 


FIG.    183.     Young    Sea-bass    (Centropristis   striatus).     Photo    from    life    by    Dr.    R.    W. 

Shufeldt. 

Question  on  the  figure. — Locate  the  pelvic  fin  and  compare  with  other 
fish  as  to  position. 


368  ZOOLOGY. 

has  a  protective  value,  as  the  discharge  is  in  some  cases  power- 
ful enough  to  paralyze  much  larger  animals  than  the  fish  itself. 
It  is  probably  useful  also  in  capturing  prey. 

Asymmetry. — In  the  flat-fishes  we  find  a  very  striking  com- 
pression from  side  to  side.  In  early  life  they  have  the  posi- 
tion normal  to  other  fish,  but  in  the  adult  stage  they  rest  and 
swim  with  the  dorso-ventral  plane  horizontal  instead  of  verti- 
cal— on  the  left  side  in  some  species  and  on  the  right  in  others. 
The  side  that  is  uppermost  becomes  pigmented  like  the  back, 
and  the  under  side  loses  its  pigment  and  becomes  white,  as  the 
belly  of  fishes  in  the  normal  position.  The  eye  which  belongs 
to  the  under  side  changes  its  position  until  it  comes  to  lie  on 
the  upper  side.  The  bones  of  the  cranium,  especially  those 
about  the  eye,  are  twisted  and  the  right  and  left  branches  of 
the  jaw  are  unequally  developed.  The  dorsal  and  ventral  fins 
become  continuous  and  the  body  tends  to  become  bilaterally 
symmetrical  in  the  new  position.  We  can  scarcely  doubt  that 
this  asymmetrical  condition  has  been  brought  about  by  the 
position  which  the  animal  takes  in  relation  to  the  environment, 
but  we  know  that  in  some  species  the  eye  begins  to  migrate 
now  before  the  fish  comes  to  lie  on  its  side. 

385.  Classification  of  Fishes. 

Subclass  I.  Elasmobranchii  (Sharks,  Dog-fishes,  Rays,  Skates). — Marine 
fishes  with  essentially  cartilaginous  skeleton;  no  operculum  or  gill-cover; 
mouth  on  the  ventral  surface  of  the  head;  heterocercal  tail;  external 
skeleton  of  placoid  scales ;  spiral  valve  in  the  intestine ;  no  air  bladder. 
The  elasmobranchs  are  regarded  by  some  as  being  the  nearest  present 
relatives  of  the  primitive  fishes.  They  occur  most  abundantly  and 
are  larger  individually  in  warm  seas.  They  are  powerful  swimmers  as 
befits  carnivorous,  preying  animals.  They  feed  on  Crustacea,  mollusca,  and 
fish. 

Subclass  II.  Gatwidei  (Ganoid  Fishes:  Sturgeon,  Gar-pike). — Fishes 
with  bony  or  cartilaginous  skeleton;  gills  covered  by  an  operculum;  exo- 
skeleton  of  ganoid  scales  or  enameled  plates;  air-bladder  present;  spiral 
intestinal  valve;  tail  either  homo-  or  hetero-cercal. 

The  group  was  very  important  in  the  early  history  of  the  earth,  and 
the  present  species  are  a  mere  remnant  of  the  former  glory  of  the  ganoids. 
They  occur  now  chiefly  in  the  rivers  and  lakes,  though  the  sturgeon  is  also 
found  in  the  sea.  North  America  is  as  well  represented  as  any  other 
region  in  the  living  species  of  this  remarkable  group. 


PISCES.  369 

Subclass  HI.  Teleostei  (Bony  Fishes). — Fishes  with  well-ossified  skele- 
tons ;  body  covered  with  cycloid  or  ctenoid  scales ;  exoskeleton  of  bony 
plates  in  the  head  region  which  become  associated  with  bones  of  the 
internal  skeleton  to  form  the  skull;  mouth  terminal  rather  than  ventral; 
gills  covered;  spiral  valve  lacking;  air-bladder  usually  present;  homo- 
cereal  tail. 

This  subclass  embraces  the  great  majority  of  the  forms  ordinarily 
known  as  fishes.  There  are  estimated  to  be  6,000  or  more  species  of 

FIG.  184. 


FIG.    184.     Long-eared    Sunfish    (Lepornis    auritus).     Adult.     Photo    from    life    by 
Dr.  R.  W.  Shufeldt. 

teleosts,  more  than  2,000  of  which  inhabit  fresh  water.  The  group  is 
variously  divided  by  different  authors  and  the  student  must  be  referred 
to  more  advanced  texts  for  fuller  classification.  The  principal  orders  are 
outlined  below. 

Pneumatic  duct  (from  air-bladder  to  intestine)   open... Order  Physostomi. 
(Carp,  cat-fish,  sucker,  salmon,  trout,  shad,  herring,  eel,  etc.) 
Pneumatic  duct  closed. 

Dorsal,  anal,  and  pelvic  fins  spiny  in  front. 

Bones  of  the  pharynx   (branchial  arches)   distinct. 

Order  Acanthopteri. 
(Perch,  sun-fish,  mackerel,  stickleback,  silverside,  etc.) 

Bones  of  the  pharynx  united Order  Pharyngognathi. 

Dorsal,  anal,  and  pelvic  fins  without  spines Order  Anacanthini. 

(Cod-fish,  haddock,  flat-fish,  etc.) 

(Two   other  teleost  orders   of  less   importance,   embracing  some 
very  peculiar  forms,  are  the  Plectognathi  (globe-fishes)  and  the 
Lophobranchii  (sea  horses,  Fig.  58;  and  pipe-fishes). 
Subclass  IV.  Dipnoi  (Lung-fishes). — Fishes  with  a  persistent  notochord 
and  the  internal  skeleton  incompletely  ossified;  soft  cycloid  scales;  spiral 
valve  in  the  intestine,  the  swim-bladder  used  as  a  lung,  the  auricle  partly 
separated  into  two  chambers,  paired  appendages  with  a  central  axis  pro- 
ducing a  flapper  rather  than  a  fin  (Fig.  174).    There  are  only  three  or  four 
25 


370 


ZOOLOGY. 


living  species,  but  these  are  especially  interesting  to  the  zoologist  from 
the  fact  that  they  may  represent  the  division  of  fishes  from  which  the 
air-breathing  vertebrates  sprang.  One  genus  (Ceratodus)  is  found  in  the 
rivers  of  Queensland;  the  second  (Protopterus)  in  the  rivers  of  southern 

FIG.  185. 


FIG.   185.     Sheepshead.     Greatly    reduced.     Photographed     from    life    by    Dr.     R. 

Shufeldt. 


W. 


FIG.    186. 


FIG.   1 86.     Young  of  the  Snowy  Grouper   (Epinephelus  niveatus).     Photo  from  life  by 
Dr.  R.  W.  Shufeldt:  American  Naturalist. 

Africa,  and  a  third  (Lepidosiren}  in  the  Amazon  in  South  America.  No 
marine  forms  are  known.  From  fossil  remains  it  is  evident  that  the 
ancestors  of  the  present  lung-fishes  were  very  much  more  widely 
distributed. 


PISCES.  371 

386.  Supplementary  Studies  for  Library  and  Field. 

1.  What  are  the  theories  as  to  the  origin  of  the  paired  fins  of  fishes? 

2.  In  what  way  do  fishes  change  their  long  axis   from  the  horizontal 
position  so  as  to  ascend  or  descend  obliquely  in  swimming? 

3.  Range  of  size  in  fishes. 

4.  Probable  origin  of  fresh-water  fishes.     What  forms  are  now  able  to 
pass  back  and  forth  from  fresh  to  salt  water? 

5.  Accumulate  data  concerning  the  habitat,  food,  breeding  habits,  dis- 
tribution, economic  importance   (with  the  reasons  therefore)    of  some  of 
the  following  fishes :    salmon,  trout,  white-fish,  sun-fish,  muskalonge,  her- 
ring, eel,  cod,  flat-fish,  mackerel,  shark,  ray,  sturgeon,  gar-pike,  bowfin.4 

6.  What  is  known  -of  the  habits  of  the  lung-fishes  calculated  to  suggest 
how  the  lung  may  be  of  value  in  preserving  the  life  of  the  animals? 

7.  Migrations  among  fishes. 

8.  Parental  care  among  fishes. 

9.  The  number  of  eggs  produced  by  various  species. 

10.  Study  figures  showing  the  embryology  of  the  salmon  or  other  bony 
fish. 

11.  The  blind  fishes  found  in  caves.    What  are  the  principal  facts  con- 
cerning them,  and  what  explanations  have  been  offered  to  account  for  their 
habits  and  modifications. 

12.  Collect  all  the  data  possible  concerning  the  flat-fishes. 

13.  Examine  all  the  figures  of  fishes  found  in  your  library,  and  make 
note  of  the  chief  points  of  variation  and  the  range  of  these. 


CHAPTER    XXL 

CLASS    II.— AMPHIBIA    (FROGS,   TOADS,    SALAMANDERS). 

« 

387.  The  amphibians  are  especially  interesting  to  the  zoolo- 
gist because  they  begin  life  as  gill-breathers   (tadpoles),  and 
later  they  replace  the  gills  by  lungs.    The  fact  that  the  amphi- 
bian in  its  individual  life  passes  from  a  fish-like  condition  to 
the  form  and  habits  of  the  higher  air-breathing  vertebrates  is 
taken  as  evidence  that  the  higher  vertebrates  have  sprung  from 
fish-like  ancestors  through   forms  similar  to  the  amphibians. 
The -change  from  gills  to  lungs  is  not  equally  striking  in  all 
the  members  of  the  group.     The  transition  from  water  to  air 
involves  important  changes  in  the  problem  of  physical  sup- 
port, of  locomotion,  and  of  respiration,  and  in  consequence,  of 
the  organs  performing  these  functions,  as  well  as  correlated 
changes  in  the  integument  and  in  the  organs  of  circulation. 
The  amphibia  were  much  more  abundant  in  earlier  geological 
times  than  at  present,  and  attained  huge  size,  whereas  the 
modern  forms,  with  a  very  few  exceptions,  are  small.     There 
are  about   nine   hundred   living  species.      The   tailless   types 
(frogs  and  toads)  are  much  the  more  numerous,  as  well  as 
more  highly  developed. 

388.  General  Characters. 

1.  Amphibia  are  Vertebrata  which  possess  gills  during  the 
larval  stage  and  lungs  in  the  adult ;  in  some  instances  the  gills 
are  retained  throughout  life. 

2.  Paired  appendages,  when  present,  conform  to  the  general 
vertebrate  type;  i.  e.,  limbs  with  digits  (typically  five),  instead 
of  fins. 

3.  Exoskeleton  of  scales  and  plates  absent;  skin  glandular. 

4.  Heart  is  three-chambered ;  two  auricles  and  one  ventricle. 

5.  A    renal-portal    and    hepatic-portal    circulation    present. 
Red  corpuscles  are  nucleated. 

372 


AMPHIBIA.  373 

6.  A  cloaca  occurs,  into  which  the  anus  and  the  ducts  from 
the  excretory  and  genital  organs  open. 

7.  Development  usually  by  a  metamorphosis.    Segmentation 
total  but  unequal. 

389.  Form. — Amphibia  differ  much  as  to  the  shape  of  the 
body.    The  newts  and  salamanders  are  elongated,  slender  and 
eel-like ;  the  frogs  and  toads  have  large,  flat  heads,  stout  trunk, 
and  strong  muscular  limbs.     Among  the  former  groups  there 
may  be  as  many  as  two  hundred  and  fifty  body  segments,  in 
the  latter  the  vertebrae  behind  the  head  are  reduced  to  ten.    The 
neck  is  usually  inconspicuous,  the  head  being  poorly  movable. 

390.  Appendages. — There  may  be  two  pairs  of  appendages, 
one  pair,  or  none  at  all.     In  most  forms  except  the  Anura 
(tailless)  the  limbs  are  small  and  weak  as  compared  with  the 
body  (Fig.  188).    The  limbs  have  a  distinct  dorsal  and  ventral 
(palmar)    surface,   as   well   as   an   anterior   and   a  posterior 
border.     The  digits  are  enumerated  from  the  anterior  border 
which  terminates  in  the  first,  or  thumb.     In  many  forms  there 
is  a  reduction  of  the  digits  on  the  anterior  appendage  from 
five  to  four.     The  digits  are  almost  universally  destitute  of 
claws. 

391.  The  skin  is  normally  soft,  and  slimy  by  reason  of  a 
glandular  secretion.     It  is  composed  of  two  layers,  epidermis 
and  dermis.     In  the  frog  the  epidermis  is  in  two  layers,  the 
outer  of  which  may  be  shed  at  intervals.     In  toads,  and  other 
forms  frequenting  dry  places,  the  epidermis  may  form  warty 
thickenings.     The  skin  is  often  highly  colored  owing  to  the 
presence  of  pigment  cells  in  the  deeper  layers.     In  some  cases 
the  tones  of  color  may  be  changed  in  accordance  with  the  sur- 
roundings, by  the  reflex  nervous  action  of  the  animal,  resulting 
from  impressions  on  the  retina  of  the  eye.     In  the  extinct 
Labyrinthodonts  external  protective  plates  were  developed  in 
the  dermis.     Minute  dermal  scales  are  found  in  some  of  the 
lowest  present  forms  ( "  blind-worms  "  ) . 

392.  The  Skeleton. — The  points  of  contrast  with  the  skele- 


374  ZOOLOGY. 

ton  of  fishes  are,  chiefly:  the  presence  of  a  sternum  (formed 
independently  of  the  ribs)  ;  the  imperfect  development  of  the 
ribs;  the  typical  limb  skeleton;  the  union  of  the  pelvic  girdle 
with  the  spinal  column;  the  closer  fusion  of  the  upper  jaw 
with  the  cranium. 

The  vertebrae  of  the  lower  forms  are  biconcave  as  in  fishes, 
in  the  higher  forms  (Anura,  and  higher  Urodela),  concavo- 
convex.  The  vertebral  column  usually  consists  of  one  cervical 
vertebra ;  a  variable  number  of  thoracic  or  abdominal  vertebrae ; 
one  sacral,  to  which  the  posterior  girdle  is  attached;  and  a 
variable  number  of  caudal  (one,  in  Anura). 

393.  Respiration. — In  early  larval  stages  the  respiration  is 
effected  wholly  by  means  of  the  skin,  and  even  after  the  de- 
velopment of  special  organs  of  respiration  the  skin  continues  to 
serve  this  function  in  a  greater  or  less  degree.     Most  amphi- 
bians have,  when  hatched,  external  gills  which  may  be  retained 
through  life  (as  in  Siren,  the  "mud-eel"),  or  may  give  place 
to  internal  gills  covered  by  a  fold  of  skin  (as  in  the  develop- 
ment of  the  frog).    Typically,  lungs  replace  both  kinds  of  gills 
in  the  adult.    The  gill  slits  do  not  exceed  three  or  four  pairs  in 
number.     Some  of  the  aquatic  forms  retain  their  gills  when 
the  lungs  are  developed,  each  method  of  respiration  supple- 
menting the  other.     Those  which  possess  lungs  alone  in  the 
adult  must  of  necessity  undergo  profound  changes  in  passing 
from  the  water-breathing  to  the  air-breathing  habit.     The 
lungs  arise  as  a  ventral  outgrowth  from  the  oesophagus  or 
pharynx.      From   the   short   trachea   the   two   sac-like   lungs 
spring.     The  walls  are  in  folds  but  the  sacs  are  simple.     In 
some  salamanders  there  are  neither  gills  nor  lungs  in  the  adult, 
respiration  taking  place  wholly  through  the  body  surfaces. 
The  frog  breathes  through  its  nostrils.    The  mouth  cavity  can 
be  increased  by  muscular  action,  thus  allowing  the  entrance  of 
air.    The  nasal  openings  are  then  closed  by  flaps  and  the  air  is 
forced  by  muscular  action  into  the  lungs. 

394.  Supplementary  Exercises  for  the  Library. — Find  as  many  dif- 
ferent types  of  respiration  as  possible  among  the  amphibians,  and  cite  ex- 


AMPHIBIA.  375 

amples.  What  forms  have  gills  only?  What  evidence  is  there  that  the 
environment  has  much  to  do  with  hastening  or  retarding  the  change  from 
gills  to  lungs?  Give  the  natural  history  of  the  Mexican  axolotl  as  far  as 
respiration  is  concerned.  Are  any  amphibia  hatched  with  lungs  at  the 
outset? 

395.  Circulation. — In  the  gill-breathing  larvae  the  circula- 
tion is  quite  similar  to  that  in  fishes  (§  378;  Fig.  178).    When 
the  gills  are  lost  and  lungs  developed,  the  arterial  arches  (Fig. 
167)  which  supply  the  gills  change  their  course,  or  suffer  de- 
struction.    This  is  an  interesting  instance  of  the  modification 
of  old  structures  to  meet  new  demands.     Coupled  with  these 
changes  we  find  the  separation  of  the  auricle  into  two  cham- 
bers— right  and  left.     The  veins  from  the  lungs  empty  into 
the  left,  and  the  systemic  veins  into  the  right  auricle.     While 
there  is  only  one  ventricle  into  which  both  the  pure  blood  from 
the  lungs  and  the  venous  blood  from  the  system  go,  it  is  so 
arranged  that  the  venous  blood  is  chiefly  returned  to  the  lungs 
and  the  purest  blood  goes  to  the  head  and  to  the  systemic 
circulation.     The  venous   circulation  is   modified  in  general 
accordance  with  the  changes  in  the  heart  and  arteries. 

396.  Supplementary   Exercise. — Compare   the   arterial   vessels   in   the 
adult  frog  with  those  in  the  fish  and  the  tadpole  stage  of  the  frog,  and 
find  what,  in  the  opinion  of  the  authors,  is  the  fate  of  each  of  the  arterial 
arches.     See  Figs.   164-167.     What  are  the  most  important  differences  in 
the  venous  circulation  in  fishes  and  in  adult  amphibians  ? 

397.  Locomotion. — In  the  lower  amphibia,  in  which  the 
appendages  are  poorly  or  not  at  all  developed,  the  muscles  of 
the  body  show  the  segmental  arrangement  seen  in  fishes,  and 
locomotion  is  effected  by  a  serpentine  or  eel-like  action  of  the 
body.     In  the  higher  forms,  especially  the  Anura,  the  limbs 
are  well  developed ;  and  the  body  muscles  lose  something  of  the 
regularity  of  their  arrangement  and  become  more  as  we  find 
them  in  the  higher  vertebrates.    The  Anura  (frogs  and  toads) 
are  especially  adapted  to  leaping  and  swimming  by  the  great 
muscular  development  of  the  hind  legs. 

398.  Exercise.— Are  there  any  special  advantages  in  the  leaping  habit 
of  motion  either  in  the  capture  of  prey  or  in  escape  from  enemies?   Verify 


376  ZOOLOGY. 

from  behavior  of  toads  and  frogs.     Can  you  find  illustrations  from  other 
groups  of  animals? 

399.  Habits  and  Habitat. — There  are  no  marine  Amphibia. 
Nearly  all  live  in  or  near  the  fresh  water  streams,  swamps,  or 
ponds,  even  in  the  adult  stage.    Some  are  good  climbers  (tree- 
toads)  ;  others  burrow.    The  tailless  forms  (Anura)  are  found 
the  world  over.     The  Urodela  belong  chiefly  to  the  northern 
hemisphere.      All    are   more   abundant    in   warmer   climates. 
Their  food  consists  largely  of  insects,  worms,  and  the  smaller 
animals.    The  larvae  even  of  carnivorous  forms  are  sometimes 
vegetable  feeders.     They  may  live  for  a  long  time  without 
food,  and  survive  the  winter  in  the  colder  latitudes  by  burrow- 
ing deep  into  the  mud  at  the  bottom  of  their  ponds,  or  other- 
wise hibernating. 

400.  Reproduction     and     Development. — The    common 
amphibia  lay  rather  large  eggs,  with  a  considerable  amount  of 
yolk  which  results  in  more  or  less  unequal  cleavage  (Fig.  n,B). 
The  eggs  are  usually  surrounded  by  a  gelatinous  material,  for 
their  protection  and  adhesion,  but  they  have  no  shell.     They 
are  almost  universally  deposited  in  the  water,  where  impreg- 
nation takes  place.     In  some  of  the  Urodela  impregnation  is 
internal.     Ordinarily  further  development  takes  place  in  the 
water  without  any  attention   from  the  parents    (frogs   and 
toads).     In  a  small  South  American  frog  (Rhino derma)  the 
male  carries  the  fertilized  eggs  in  his  vocal  sacs  until  hatched ; 
in  one  of  the  tree-frogs  from  South  America  the  female  has  a 
pouch  on  the  back  in  which  the  eggs  are  stored  and  hatched ;  in 
the  Surinam  toad  the  eggs  are  placed  by  the  male  on  the  back 
of  the  brooding  female,  where  they  become  surrounded  by 
spongy  tissue.     In  these  pits  they  hatch  at  once  into  the  adult 
form  without  having  external  gills.     This  is  of  course  a  suc- 
cessful adaptation   (by  eliminating  the  metamorphosis)   to  a 
completely  aerial  habit.     From  this  group  we  have  beautiful 
illustrations  of  unequal  cleavage  of  the  ovum,  of  which  the 
student  should  have  the  opportunity  of  seeing  figures  in  more 


377 


FIG.    187.     The    metamorphosis   of    the    Frog.      (After    Brehm.)     Numbers   indicate   the 

sequence. 

Questions  on  the  figures. — How  much  of  the  egg  is  really  ovum? 
What  are  the  changes  which  take  place  in  passing  through  the  various 
stages?  In  what  order  do  the  legs  appear?  How  is  respiration  affected 
after  stage  6?  After  stage  n?  What  is  proven  by  the  collecting  of  the 
tadpoles  as  shown  in  3?  How  do  they  retain  their  position. 

FIG.  188. 


FIG.   1 88.     Tailed    Amphibians.     From    Nicholson,    after    Mivart.     A,    Siren;    B,    Am- 
phiuma  tridactyla;  C,  Necturus. 

Questions  on  the  figures. — Compare  these  three  types  and  note  all  the 
chief  differences  of  external  structure.  Compare  also  with  figures  you 
may  be  able  to  find  of  other  Urodela. 


FIG.   189.     Frog    (Rana).     Photo    from    life    by    J. 


Folsom. 


Questions  on  the  figure. — What  is  the  round  object  behind  the  eye? 
What  elements  in  the  resting  position  of  the  frog  put  him  in  readiness 
for  a  quick  spring? 


FIG.  190. 


FIG.    190.     The  Common  Toad  (Bufo  lentiginosus).     Photo  from  life  by  Folsom. 

Questions  on  the  figure. — Compare  with  the  figure  of  the  frog  and  note 
points  of  external  similarity  and  difference.  What  do  you  know  of  the 
habits  of  the  toad,— as  to  feeding,  egg-laying,  etc.?  Where  do  they  spend 
the  winter?  What  of  their  development? 


AMPHIBIA.  379 

extended   works.      If   possible   the   cleaving   eggs   should   be 
studied. 

401.  Special  Exercises. — Describe  the  life  history  and  the 
stages  in  the  metamorphosis  of  the  frog.     (Fig.  187.)     What 
larval  organs  disappear?    What  new  organs  are  introduced? 
Compare  other  amphibians  as  to  the  degree  and  facts  of  meta- 
morphosis. 

402.  Classification  of  Amphibia. 

Order  I.  Urodela. — Amphibia  with  tails  persistent  throughout  life;  body 
elongated;  usually  two  pairs  of  appendages  (sometimes  only  the  anterior 
are  present),  which  may  be  poorly  developed. 

The  principal  suborders  are  : 

1.  Perennibranchiata,  in  which  the  gills  persist  throughout  life    (Nec- 
turus  or  water-dog,  Siren  or  mud-eel,  and  certain  blind  forms  found  in 
caves). 

2.  Derotremata,  losing  the  gills  in  the  adult  but  retaining  a  spiracular 
opening  in  the  side  of  the  neck  which  represents  the  gill-slit.     (Examples: 
"  Congo-snake  "  of  the  gulf  states,  giant  salamander  of  Japan.) 

3.  Myctodera,  which  lose  all  traces  of  water-breathing.      (Examples: 
Newts,  salamanders,  etc.) 

Order  II.  Anura. — Amphibia  in  which  the  tail  is  absorbed  in  the  adult 
condition,  if  present  in  the  embryo.  Two  pairs  of  appendages,  the  posterior 
of  which  are  well  developed.  Undergo  a  metamorphosis  in  which  the 
larvae  usually  have  the  "  tadpole "  form,  with  gills  and  tail  but  without 
appendages.  All  traces  of  gills  lost  in  the  adult.  The  Anura  embrace  the 
Bufonidae  or  common  toads,  the  Ranidae  or  common  frogs,  the  Hylidae  or 
tree-toads,  and  other  less  common  families.  The  Anura  include  the  ma- 
jority of  the  species  of  Amphibia. 

Order  HI.  Gymnophiona. — Amphibia  with  neither  legs  nor  tail;  body 
worm-like ;  no  gills  nor  gill-slits  in  the  adult ;  eyes  more  or  less  degenerate. 
Scales  are  present  in  the  skin.  Represented  by  the  so-called  blind-worms 
of  tropical  countries. 


CHAPTER    XXII. 
CLASS  III.— REPTILIA  (LIZARDS,  CROCODILES,  TORTOISES,  SNAKES). 

LABORATORY  WORK. 

403.  Specimens  of  reptiles  are  scarcely  abundant  enough  to 
serve  as  satisfactory  laboratory  types  for  elementary  classes, 
but  instructive  comparisons  may  be  made  by  single  students  or 
by  groups  of  students.    These  results  should  be  reported  to  the 
class. 

Prepare  three  parallel  columns,  one  for  the  lizard,  one  for 
the  snake,  and  one  for  the  turtle.  Select  a  specimen  of  each 
and  compare  them  with  regard  to  their  haunts ;  habits ;  food ; 
general  form  of  body;  appendages,  number,  position,  joints, 
digits;  covering;  manner  of  locomotion. 

404.  Special  Topics  for  Investigation  in  the  Laboratory. 

1.  Are  reptiles  warm  or  cold  blooded?    Your  evidences? 

2.  What  are  the  differences  between  the  scales  of  snakes  and  of  fishes? 

3.  In  what  various  ways  is  the  tail  of  reptiles  used  as  an  organ?    How 
is  the  tail  to  be  distinguished  from  the  rest  of  the  body? 

4.  What  special  senses  do  reptiles  possess?    What  are  your  evidences? 
What  peculiarities  have  the  organs  of  sense? 

5.  What  peculiarities  do  the  internal  organs  of  the  snake  have  which 
seem  to  be  correlated  with  the  slender,  elongate  form  of  the  animal? 

6.  What    species    of   snakes,    turtles,    and    lizards    are    found    in   your 
locality?   Report  on  the  special  habits  of  each  species  in  so  far  as  you  can 
determine  them  by  observation.    Supplement  by  reference  to  authorities. 

DESCRIPTIVE  TEXT. 

405.  The    Reptilia   differ    from   the   vertebrates    we   have 
hitherto  studied  in  the  fact  that  at  no  period  of  life  do  they 
possess  gills.    They  agree  with  the  lower  forms  in  being  cold- 
blooded and  in  the  incomplete  separation  of  the  heart  into  right 
and  left  compartments  (except  in  the  crocodiles).    They  are,  in 
addition  to  their  air-breathing  habit,  similar  to  the  birds  and 
mammals  in  possessing  the  protective  embryonic  membranes 

380 


REPTILIA.  381 

known  as  the  ainnion  and  allantois  (see  §  415),  the  latter  of 
which  is  important  in  embryonic  respiration,  that  is,  before 
hatching  or  birth.  The  group  reached  its  culmination  in 
numbers,  variety  and  size  in  the  Mesozoic  age.  So  true  is  this 
that  the  Mesozoic  is  called  the  "  Age  of  Reptiles."  Those  we 
have  at  present  are  to  be  looked  upon  as  specialized  and,  in 
some  instances  (snakes)  perhaps,  degenerate  remnants  of  the 
first  vertebrate  class  wholly  to  give  up  breathing  by  means  of 
gills.  In  the  Mesozoic  era  there  were  immense  swimming, 
fish-like  forms  (ichthyosaurs  and  plesiosaurs)  which  ruled  the 
seas;  powerful  terrestrial  dinosaurs,  often  walking  on  their 
hind  legs,  and  including  the  largest  land  animals  known  to 
have  lived;  and  others,  with  membranous  wings  like  the  bat, 
the  first  vertebrates  to  learn  the  art  of  flying  (Fig.  193).  With 
the  exception  of  a  few  marine  turtles,  the  boas  and  pythons, 
and  the  alligators  and  crocodiles,  the  living  species  are  for  the 
most  part  small  animals. 

406.  General  Characteristics. 

1.  Reptilia  are  usually  covered  with  scales  or  plates  derived 
from  the  dermis  (bony),  or  the  epidermis  (horny),  or  from 
both. 

2.  The  (3-5)  digits  when  present  are  provided  with  claws. 

3.  The  vertebrae   are  concavo-convex,   usually   concave   in 
front  and  convex  behind. 

4.  The  heart  is  three  chambered; — that  is,  the  auricles  are 
completely  separated,  but  the  ventricles  are  only  partially  so 
except  in  the  Crocodilia. 

5.  There  are  two  aortic  arches,  a  right  and  a  left,  in  the 
adult. 

6.  Gills  do  not  occur  at  any  period. 

7.  Reptiles  are  chiefly  oviparous;  the  eggs  are  large,  well 
supplied  with  yolk,  and  protected  by  a  leathery  shell. 

8.  The  embryonic  membranes, — amnion  and  allantois — first 
make  their  appearance  in  this  group. 


32  ZOOLOGY. 

407.  The  Reptiles  are  very  diverse  in  form.     Perhaps  the 
lizards  may  be  taken  as  typical,  with  cylindrical  body,  more 
or  less  distinct  head  and  neck,  distinct  tail,  and  usually  two 
pairs  of  appendages,  each  possessing  five  digits  armed  with 
claws.     They  are  mostly  small  animals,  though  one  species  is 
known  to  attain  a  length  of  five  feet.     The  crocodiles  and 
alligators  are  similar  in  shape  but  much  larger.     The  turtles 
and  snakes  are  most  widely  different  from  the  type  and  must 
be  regarded  as  much  specialized,  or  even  degenerate,  forms. 
The  turtles  have  sought  protection  by  means  of  a  bony  box, 
and  are  ill  adapted  for  motion  either  on  land  or  water.    Snakes, 
on  the  other  hand,  elongated  and  devoid  of  appendages,  are 
among  the  most  rapid  and  graceful  of  animals  in  their  motions. 
The  long  tapering  body  is  a  successful  prehensile  organ.    Some 
of  the  lizards  agree  with  the  snakes  in  lacking  legs. 

408.  Covering. — The  external  covering  in  reptiles  is  in  the 
form  of  scales  or  plates  formed  by  the  epidermis,  or  the  dermis, 
or  both.     That  deposited  by  the  epidermis  is  horny  and  that 
by  the  dermis,  bony.     In  snakes  and  many  lizards  the  scales 
are  epidermal  and  may  be  periodically  shed  and  renewed.    The 
scales  usually  differ  in  shape  and  size  in  different  parts  of 
the  body.     In  turtles  and  their  allies  the  horny  constituent, 
which  is  illustrated  by  the  "  tortoise  shell "  of  commerce,  is  in 
the  form  of  plates  and  is  reinforced  by  bony  dermal  plates 
beneath.    The  latter  do  not,  in  the  adult  at  least,  correspond  in 
number  and  size  with,  the  former,  but  are  closely  associated 
with  the  bones  of  the  internal  skeleton.    In  crocodiles  the  der- 
mal scales  correspond  in  general  with  the  epidermal. 

409.  Internal  Skeleton. — The  vertebral  column,  except  in 
the  snakes  and  snake-like  lizards,  shows  the  customary  regions 
(see  §  341).     In  the  limbless   forms  only  two  regions  are 
recognized, — the   pre-caudal   which   bear   the   ribs,    and   the 
caudal  or  tail  vertebrae.     The  vertebrae  are  usually  concave  in 
front  and  convex  behind,  thus  making  a  kind  of  ball-and- 
socket  joint.     In  snakes  the  number  of  vertebrae  is  very  large. 


REPTILIA.  383 

No  sternum  occurs  in  turtles  and  snakes.  When  present,  as 
in  lizards  and  crocodiles,  it  is  formed  in  connection  with  the 
ventral  end  of  the  ribs. 

The  skull  articulates  with  the  first  vertebra  by  one  surface 
(condyle)   instead  of  two  as  in  mammals.     The  lower  jaw 


pmx 


de~~ 


FIG.  191.  Skull  of  Rattlesnake  (Crotalus  durissus).  From  Nicholson,  after  Huxley. 
ar,  articular  portion  of  lower  jaw;  de,  dentary  portion;  bo,  basi-occrpital ;  mx,  maxilla, 
bearing  poison  fang;  no,  nasal;  pi,  palatine,  the  front  end  being  represented  by  a  dotted 
line  as  though  seen  through  the  maxilla;  pmx,  premaxilla;  po,  post  frontal;  pr,  pre- 
f rental;  pt,  pterygoid;  qu,  quadrate;  sq,  squamosal;  tr,  transverse  bone. 

Questions  on  the  figure. — Which  bones  bear  teeth?  Which  are 
cranial  and  which  facial  bones?  What  bones  do  you  find  common  to  the 
snake  and  the  fish  (Fig.  177)?  How  do  they  differ  in  the  two  forms? 
What  is  the  function  of  the  quadrate?  How  does  it  differ  in  the  different 
groups  of  Vertebrates  ? 

articulates  indirectly,  by  means  of  the  quadrate  bone,  with  the 
skull.  This  gives  a  very  movable  jaw  and  in  the  snakes  espe- 
cially, increases  the  caliber  of  the  throat  (Fig.  191).  The 
skull  is  more  compactly  fused  and  completely  ossified  than 
among  the  Amphibia.  The  ulna  and  radius  and  the  tibia  and 
fibula  are  not  fused  as  in  the  frog.  Rudiments  of  the  pelvic 
girdles  are  found  in  some  snakes,  although  the  limbs  are 
wanting. 


384 


ZOOLOGY. 


FIG.  192. 


pt.  z 


t.  p. 


FIG.  192.  Vertebrae  of  a  Reptile  (after  Huxley).  A,  anterior  view;  B,  posterior 
view  of  the  vertebra  in  front  of  A.  The  surface  of  A  fits  against  the  surface  of  B. 
c,  centrum,  which  is  convex  in  B,  fitting  into  a  concavity  in  A;  n.s.,  neural  spine;  pr.z., 
pre-zygapophyses,  or  anterior  articular  facets,  which  fit  against  pt.s.,  post-zygapophyses; 
t.p.,  transverse  processes;  s.s.,  a  wedge-like  articular  face  on  the  neural  arch  designed  to 
fit  into  2. a.,  a  depression  on  the  posterior  face  of  the  neural  arch  of  the  vertebra  in 
front  (B). 

Questions  on  the  figure. — Try  to  form  a  clear  picture  of  the  relations 
of  the  articulating  surfaces  of  the  vertebne  and  indicate  the  possible  ad- 
vantages of  the  arrangements.  Where  is  the  neural  cavity?  Where  do 
the  ribs  articulate?  What  is  the  gain  in  muscular  attachments  from  the 
numerous  bony  outgrowths  on  the  vertebrae? 

410.  Respiration. — Functional   gills   never   occur,   though 
gill-slits  are  partly  developed  in  the  embryo  only  to  close  again 
before  hatching.     The  trachea  is  elongated  and  is  supported 
by  cartilaginous  rings  as  in  the  higher  forms.     It  divides  into 
two  bronchi,  each  of  which  passes  to  a  spindle-shaped  sac — 
the  lung — which  is  much  simpler  in  its  lobings  than  those  of 
birds  and  mammals.    In  the  snakes  one  lung  (the  left)  is  much 
reduced  or  even  altogether  aborted.     This  is  an  adaptation  to 
the  narrow  elongated  body  cavity.    The  ribs  when  present  and 
the  muscles  acting  on  them  are  the  prime  agents  in  breathing. 

41 1.  Circulation. — In  reptiles  the  right  and  left  auricles  are 
entirely  distinct  but,  with  the  exception  of  the  Crocodilia,  the 
ventricles  are  only  partially  so.     Yet  in  those  forms  in  which 
the  pure  blood  of  the  left  auricle  and  the  impure  blood  of  the 
right  partially  mingle  in  the  ventricle,  the  arrangement  is  such 


REPTILIA.  385 

that  the  purest  blood  goes  to  the  brain  and  the  least  pure  to 
the  lungs  (see  Fig.  168).  Two  aortic  arches  unite,  giving  rise 
to  the  dorsal  aorta.  In  the  reptiles  and  higher  groups  of 
vertebrates  the  renal-portal  circulation  (see  Fig.  178,  r.p.) 
ceases  to  be  of  much  importance,  but  the  hepatic  portal  is 
increasingly  important.  The  red  corpuscles  are  elliptical  and 
possess  nuclei. 

412.  Nervous  System  and  Special  Sense  Organs. — The 

brain  is  not  large  in  the  reptiles,  but  is  rather  more  highly 
developed  than  in  the  Amphibia.  This  is  especially  true  of  the 
cerebral  hemispheres.  The  usual  senses  are  represented.  The 
rather  larger  eyes  are  provided  with  movable  eyelids  except 
among  the  snakes,  in  which  a  permanent  transparent  membrane 
covers  the  eye.  In  some  reptiles  (lizards)  there  is  a  remnant 
of  a  median  eye  which  is  hopelessly  degenerate  in  the  adult. 
It  is  in  connection  with  the  pineal  body  in  the  second  division 
of  the  brain. 

413.  Habits. — The    reptiles    are   best    represented    in    the 
tropical  regions.     The  larger  types,  as  the  crocodiles,  python, 
boa  are  almost  confined  to  the  warm  zones,  especially  of  South 
America,  Africa  and  Asia.     Numerous  smaller  representatives 
of  the  lizards,  snakes,  and  turtles  are  found  in  temperate  lati- 
tudes.    These  usually  undergo  a  period  of  hibernation  during 
the  cold  season.    This  habit  of  hibernating  and  seeking  warmer 
climates  seems  related  to  the  cold-blooded  condition.    The  heat- 
producing  qualities  of  the  animals  are  not  equal  to  the  task  of 
maintaining  activity  during  extreme  cold.     The  variation  of 
temperature  is  of  course  a  more  serious  problem  to  terrestrial 
animals  than  to  aquatic  types.     Although  air-breathers,  very 
many  of  the  group  are  aquatic,  as  the  turtles,  crocodiles,  and 
many  snakes.    The  lizards  are  almost  without  exception  terres- 
trial.   Nearly  all  prey  on  other  animals ;  the  smaller  on  worms, 
insects,  and  eggs  of  various  kinds,  and  the  larger  on  birds, 
fish,  amphibia,  and  mammals.     The  land  tortoises  are  vege- 
table feeders. 

26 


386 


ZOOLOGY. 


Reptiles,  especially  the  snakes,  have  a  bad  reputation,  yet 
there  is  no  doubt  that  their  dangerous  qualities  are  much 
exaggerated  in  popular  opinion.  The  lizards  are  almost 
wholly  non- venomous  and  the  majority  of  the  common  snakes 
of  this  country  are  also  harmless.  The  principal  dangerous 
snakes  are  the  cobra  of  the  East  Indies  where  nearly  25,000 
deaths  were  caused  by  serpents  in  1899;  the  vipers  of  Europe; 

FIG.  193. 


FIG.   193.     Rhamphorhynchus   merensteri, — a    restoration    of   an    extinct    flying    Reptile. 
From   the   Cambridge   Natural   History,    after   Geikie. 

Questions  on  the  figure. — In  what  respects  does  a  form  like  this 
differ  in  external  appearance  from  a  bird?  From  a  bat?  What  skeletal 
structures  would  a  palaeontologist  need  to  find  in  order  to  believe  that 
an  extinct  form  had  the  power  of  flight? 

the  rattle-snake,  water-moccasin,  and  copperhead  of  our  own 
country.  The  venom  serves  the  snake  both  as  a  means  of 
defense  and  of  paralyzing  its  prey.  Many  forms  which  are  not 
poisonous  assume  bodily  attitudes  similar  to  those  of  the 
poisonous  species.  This  is  known  as  mimicry,  and  is  a  means 
of  protection.  The  dangerous  species  are  being  rapidly  ex- 
terminated by  man. 

414.  Special  Exercises. — Find  data  concerning  hibernation  in  reptiles 
and  other  vertebrates:  its  object  and  advantages;  preparation;  place;  de- 
gree in  which  vitality  is  suspended  during  the  process,  etc. 

Describe  the  poison  apparatus  of  venomous  snakes.  What  is  the 
homology  of  the  fang?  Of  the  gland? 

How  do  different  snakes  capture  their  prey?  How  prepare  it  for 
swallowing? 


REPTILIA.  387 

Describe  and  attempt  to  explain  the  motion  of  snakes  from  actual 
observation  :  in  water ;  on  land. 

415.  Reproduction  and  Development. — The  ova  escape 
from  the  two  ovaries  into  the  body  cavity.  As  in  the  Amphibia 
the  inner  end  of  the  oviduct  opens  well  forward  in  the  body- 
cavity.  The  ova  enter  the  oviducts,  and  during  the  descent  are 
fertilized.  After  fertilization  the  glands  in  the  walls  of  the 
oviducts  add  albumen  and  shell  structures,  as  in  the  birds. 
The  eggs  require  a  period  of  incubation  which  usually  occurs 
outside  the  body,  though  some  lizards  and  snakes  retain  the 
eggs  in  a  special  portion  of  the  oviduct  until  the  embryo  is 
hatched.  Many  forms  deposit  their  eggs  in  the  warm  sand 
or  earth  or  in  decaying  rubbish  heaps,  where  the  abundant 
heat  is  favorable  for  the  developing  young. 

Much  yolk  is  present  in  the  egg  and  segmentation  is  partial, 
being  confined  to  a  disc.  The  germinal  layers  and  the  im- 
portant organs  develop  about  the  axis  of  this  disc,  the  outer 
margins  of  which  spread  over  the  whole  yolk  in  the  form  of  a 
sac  designed  to  nourish  the  embryo.  The  details  of  the  growth 
are  entirely  too  complicated  for  statement  here.  Two  important 
embryonic  membranes — the  amnion  and  allantois — appear  for 
the  first  time  (see  also  §  431).  The  amnion  consists  of  folds 
of  the  blastodermic  disc  which  arise,  surrounding  the  embryo 
at  its  margin.  These  folds  grow  dorsally  over  the  embryo  and 
ultimately  fuse  to  enclose  a  space  which  becomes  filled  with 
fluid.  The  amnion  folds  include  both  ectoderm  and  mesoderm. 
It  is  protective  in  function  ( Fig.  205,  am) .  The  cavity  between 
the  two  layers  of  the  amnion  is  an  outgrowth  of  the  coelom. 
The  allantois  arises  as  a  fold  from  the  posterior  portion  of  the 
digestive  tract,  and  is  made  up  of  entoderm  and  mesoderm. 
It  finally  surrounds  not  merely  the  embryo  but  the  yolk  on  the 
ventral  side,  and  being  well  supplied  with  blood  vessels  is  most 
important  in  supplying  the  embryo  with  oxygen.  In  this  and 
in  other  features  the  reptiles  show  a  close  kinship  with  the 
birds. 


388  ZOOLOGY. 

416.  Classification  of  Reptiles. 

Order  I.  Chclonia  {Turtles  and  Tortoises). — The  Chelonia  are  reptiles 
with  short,  flattened  or  dome-shaped  bodies  enclosed  in  a  case  formed 
by  a  dorsal  shield  (carapace)  and  a  ventral  (plastron).  The  jaws  are 
covered  with  a  horny  case  and  are  destitute  of  teeth.  The  quadrate  bone 
is  firmly  fused  to  the  cranium.  The  sternum  is  absent.  Turtles  seem 
rather  more  common  in  the  northern  hemisphere.  The  largest  species  are 
marine  and  may  attain  a  weight  of  half  a  ton.  Some  live  in  fresh  water 
and  others  on  land.  The  flesh  of  some  species  is  much  prized  for  food. 
The  green-turtle  of  the  Atlantic  coast  is  one  of  the  choicest,  its  flesh 
being  much  used  for  soups.  The  large  hawkbill-tnrtle  of  the  tropical  seas 
furnishes  "  tortoise-shell,"  used  in  combs  and  other  ornaments.  The 
shells  of  the  leather-back  and  other  "  soft-shelled  "  turtles  are  not  com- 
pletely ossified.  The  "  snappers "  are  ferocious  animals,  the  big  snapper 
of  the  Southern  states  being  particularly  vicious. 

FIG.  194. 


FIG.   194.     Common  Box  Tortoise    (.Cistudo  Carolina).     Photographed  from  life  by 
Dr.  R.  W.  Shufeldt. 

Order  II.  Lacertilia  (Lizards). — Reptiles  in  which  the  body  is  usually 
covered  with  small  scales.  Two  pairs  of  limbs  are  ordinarily  present ; 
but  either  or  both  may  be  wanting.  The  quadrate  bone  is  somewhat 
movable.  The  teeth  are  not  in  sockets  of  the  jaw.  Sternum  present. 
The  cloacal  opening  is  transverse. 

The  Lacertilia  include,  beside  the  types  commonly  known  as  lizards, 
the  chameleons,  horned-toads,  and  the  glass  snake — a  legless  lizard.  They 
subsist  largely  on  insects  and  the  eggs  of  other  animals.  Only  one  species 
is  known  to  be  poisonous — the  "  Gila  monster "  of  New  Mexico  and 


389 


FIG.    195.     Swift  Lizard    (Sceloporus  undulatus).     Adult.     Photographed   from   life   by 

Dr.  R.  W.  Shufeldt. 


southward.  The  glass  snake  possesses  in  a  high  degree  a  power  more 
or  less  common  among  lizards — of  breaking  loose  from  the  tail  when 
struck  or  held  by  that  organ.  In  some  species,  at  least,  a  new  tail  may  be 
regenerated.  Most  lizards  are  terrestrial,  though  a  few  are  aquatic. 

Order  III.  Ophidia  (Snakes).— Reptiles  with  elongated  bodies  covered 
by  fold-like  epidermal  scales  which  may  be  shed  as  a  single  "  cast."  Limbs 
are  wholly  wanting.  The  mouth  is  capable  of  great  extension  on  ac- 
count of  the  great  movability  of  the  quadrate  and  other  bones.  Teeth 
are  numerous  and  fused  (not  in  sockets)  to  the  bones  bearing  them. 
Sternum  wanting.  There  are  no  movable  eyelids.  The  tongue  is  pro- 
trusible,  and  is  doubtless  much  used  as  an  organ  of  touch. 

Snakes  are,  like  lizards,  partial  to  warm  climates,  but  are  also  found  in 
temperate  latitudes.  Most  are  terrestrial,  but  some  take  to  water  readily; 


39° 


ZOOLOGY. 
FIG.  196. 


FIG.    196.     Flying  Lizard  (Draco  volitans).     From  Nicholson. 


FIG.  197. 


FIG.   197.     Dorsal  view  of  a  "  Gila  Monster"    (Heloderma  suspectum).     Photographed 
by    Dr.    R.   W.    Shufeldt. 


REPTILIA. 

FIG.  198. 


391 


FIG.   198.     Common  Garter   Snake    (Eutcenia  sirtalis).     Photographed   from  life  by 
Dr.    R.  W.   Shufeldt. 


FIG.  199. 


FIG.    199.      Blotched      King      Snake      (Lampropeltis     rhombomaculatns).     Photographed 
from  life  by  Dr.  R.  W.  Shufeldt. 

Questions  on  the  figure. — What  structural  adaptations  have  the 
snakes  which  tend  to  take  the  place  of  appendages?  Illustrate  your  con- 
clusions by  citing  examples. 


392  ZOOLOGY. 

and  there  are  some  which  never  come  to  land.  These  and  some  land 
forms,  bring  forth  their  young  alive.  Many  snakes  are  beautifully 
and  characteristically  colored.  In  some  instances  the  coloration  is  deemed 
to  be  protective. 

Order  IV.  Crocodilia  (Crocodiles,  Alligators,  etc.). — Fresh-water  rep- 
tiles with  elongated  bodies  bearing  two  pairs  of  well-developed  appendages. 
The  skin  is  armed  with  dermal  bony  scales  or  scutes  covered  by  epi- 
dermal scales.  Teeth  occur  in  sockets.  The  quadrate  is  immovable  and 
the  sternum  is  present.  The  adult  heart  is  completely  divided  into  right 
and  left  halves.  The  cloaca  opens  by  a  longitudinal  slit.  Here  are 
included  the  gavial  of  the  Ganges,  tne  crocodiles  of  the  Nile  and  of  tropical 
America,  and  the  alligator  of  America.  They  are  somewhat  sluggish 

FIG.  200. 


FIG.  200.     Head  of  the  American  Alligator  {Alligator  Mississippiensis).     From  Eckstein. 

animals,  but  when  hungry  will  attack  with  sutcess  the  larger  mammals 
or  man.  They  may  attain  a  length  of  twenty  feet  or  more.  Crocodilia 
are  chiefly  aquatic,  though  they  rest  on  the  shore,  and  deposit  their  eggs 
in  the  sand  where  they  hatch. 

There  are  numerous  orders  of  extinct  reptiles  which  show  close  rela- 
tionship with  the  fishes,  amphibians,  and  birds  of  early  geological  times. 
This  is  merely  another  way  of  saying  that  the  early  Reptilia  and  the 
other  vertebrates  were  much  more  generalized  in  their  characteristics  and 
less  differentiated  than  those  of  the  present. 

.     417.  Supplementary  Topics  for  Investigation. 

1.  Have  the  venomous   snakes   any  characteristic  appear- 
ance? 

2.  Report  on  the  habits  of  the  rattle-snake.     Whence  the 
structure  giving  rise  to  the  name?     The  nature  of  the  fang 
and  the  poison  gland. 

3.  What  is  the  degree  of  activity  and  strength  of  the  rep- 
tiles and  cold-blooded  animals,  as  compared  with  the  warm- 
blooded? 


REPTILIA. 


393 


4.  The  characteristics  of  the  principal  groups  of  geological 
reptiles. 

5.  The  snake  in  myth. 

6.  The  various  methods  of  capturing  prey  among  reptiles. 

7.  The    facts    concerning    incubation    and    care    of    young 
among  reptiles. 

8.  The   development   of    the   amnion    and   allantois.      See 
figures  in  reference  texts. 

9.  Library  exercise :  What  reptiles  are  viviparous  ?   Can  you 
find  any  special  reasons  assigned   for  this  in  the  particular 
cases  ? 


CHAPTER    XXIII. 
CLASS  IV.— AVES   (BIRDS). 

418.  Laboratory   and   Field    Studies. — Each    student   o: 
group  of  students  should  be  encouraged  to  select  one  or  more 
species  of  birds  and  to  study  their  habits  and  external  struc- 
ture in  the  light  of  the  following  outline. 
I.  Habits  and  Activities. 

Haunts  and  feeding  habits. 

Social  habits ;  solitary  or  gregarious  ? 

Mating  habits;  monogamous  or  polygamous?  Degree  of 
sexual  dimorphism? 

Determine  and  describe  its  powers  of  song.  Are  they 
equally  developed  in  all  individuals  of  the  species  ?  Can 
you  cite  any  evidence  that  the  power  is  of  any  use  to  the 
animals  ? 

Nesting  habits;  number  of  eggs,  their  size  and  other  char- 
acters. Is  their  color  of  any  conceivable  use?  Which 
sex  incubates  the  eggs?  Condition  of  the  young  at 
hatching  and  the  care  given  by  the  parents  to  the  young. 

Migrations.  Are  there  any  evidences  of  winter  (or  other 
important)  migrations?  If  not,  how  is  the  winter  spent? 
If  so,  at  what  time  does  it  occur  ?  When  does  the  species 
return?  Any  other  known  facts. 

Power  and  peculiarities  of  flight.  Other  modes  of  locomo- 
tion? Is  the  power  of  perching  well  developed? 

-  What  are  its  relations  to  the  other  animals  of  the  locality? 
Has  it  any  enemies?  Is  it  hostile  to  any  species  of 
animals  ? 

What  is  its  abundance  or  scarcity  in  your  locality?     Can 

you  assign  any  explanation  of  the  facts  observed? 
II.  General  External  Appearance. 

Regions  of  the  body:  head,  neck,  trunk,  limbs. 

394 


AVES.  395 

Head:  beak,  mouth,  nares,  eyes  (how  many  lids?),  ears. 

Neck :  length,  natural  position,  flexibility,  etc. 

Wings :  arm,  forearm,  hand. 

Legs :    thigh,    shank,    foot.     Where    is   the   heel  ?     Evi- 
dences?    Note   further   arrangement   and   number   of 
digits ;  form  of  the  claws ;  covering  of  the  tarso-meta- 
tarsns. 
Covering  of  the  body.     Compare  the  color  of  all  visible 

parts. 

Select  feathers  from  various  parts  of  the  body;  study  as 
a  type  one  of  the  large  wing  feathers,  noting:  shaft 
(quill  and  rachis),  vane  (barbs  and  barbules). 

Compare  the  other  feathers  selected  with  this  one.  What 
would  you  suggest  as  the  prime  function  of  each  kind  ? 

Arrangement  of  the  feathers. 

On  wings:  rcmiges  (large),  primary   (on  hand)   and 

secondary  (on  forearm)  ;  coverts. 
On  tail :  rectrices,  number  and  arrangement ;  coverts. 
On  body  (dip  in  hot  water  and  pluck)  :  note  the  pits 
which    have    borne    the    feathers;    arrangement    of 
these.     Are  they  uniformly  distributed  over  the  en- 
tire body? 
Sketch  the  plucked  bird,  studying  more  carefully  the  regions 

already  noted.     Locate 

Openings :  mouth,  nares,  ears,  cloaca. 
Queries : 

Is  there  any  connection  between  the  closing  of  the  toes 
and  the  flexing  of  the  leg?  Has  this  any  use  to  the 
animal  ? 

Which  digits  are  represented?  Are  they  ecrually  devel- 
oped? 

Which  digit  is  turned  backward?  How  is  this  deter- 
mined ? 

Is  there  a  tongue?  Are  there  teeth?  Do  the  nostrils 
communicate  with  the  mouth? 

What  do   you  consider   the   function  of  the  nictitating 


396  ZOOLOGY. 

membrane?  Are  the  eyes  movable?  Do  they  view 
the  same  field? 

Do  the  two  together  cover  the  entire  field  of  view? 

How  much  external  ear  is  present  ? 

Are  the  scales  homologous  with  feathers?  (See  refer- 
ence texts.) 

DESCRIPTIVE  TEXT. 

419.  Birds  must  be  looked  upon  as  sharing  with  mammals 
the  first  place  in  the  animal  kingdom.     Even  the  mammals  as 
a  class  are  not  so  highly  specialized  in  structure  and  in  habits 
as  the  birds.    Their  most  striking  features  of  specialization  are 
connected  with  the  demands  of  aerial  life  which  they  have  so 
successfully  met.      They   share   with   the   insects, — the   most 
specialized  of  the  invertebrate  phyla, — the  most  perfect  de- 
velopment of  the  power  of  flight  found  among  animals.     It 
follows  from  their  high  degree  of  specialization  that  they  are 
among  the  most  easily  recognized  of  the  vertebrates.     The 
earliest  geological  traces  of  birds  show  that  they  are  closely 
linked  with  reptiles  in  their  origin,  and  the  modern  birds  pre- 
serve many  interesting  likenesses  to  the  reptiles.     Some  of 
these  are  seen  in  the  scales  on  the  shank  and  feet  of  birds ;  in 
the  habit  of  laying  large,  well-nourished  eggs  which  hatch  out- 
side the  body;  in  the  structure  of  the  egg  and  its  mode  of 
cleavage;  the  peculiarities  of  the  ankle  joint;  in  the  presence 
of  a  cloaca. 

420.  General  Characteristics  of  Birds. 

1.  The  Aves  are  vertebrates  in  which  the  epidermic  out- 
growths usually  take  the  form  of  feathers  (or  scales  in  special 
regions,  as  the  feet). 

2.  The  anterior  appendages  in  the  majority  of  forms  are 
modified  for  flight.     Associated  with  this  is  a  large  de^jelop- 
ment  of  the  pectoral  muscles  and  the  bones  to  which  they  are 
attached. 

3.  A  single  occipital  condyle. 

4.  The  heart  is  completely  four-chambered;  only  one  (sys- 


AVES.  397 

temic)  aortic  arch  which  turns  to  the  right;  red  corpuscles  oval 
and  nucleated.  High  bodily  temperature,  100°  to  no0  F. 
Renal  portal  circulation  almost  wanting. 

5.  Some  of  the  bronchial  tubes  terminate  in  air  spaces  (not 
true  lung  tissue)  located  in  various  parts  of  the  body.     These 
communicate  with  air  cavities  in  some  of  the  bones. 

6.  The  parts  of  the  skeleton  are  much  fused.    There  are  no 
teeth,  the  jaw  being  sheathed  by  a  horny  product  of  the  epi- 
dermis (beak). 

7.  The  right  ovary  and  oviduct  are  aborted  or  rudimentary. 

8.  All   are   oviparous;   yolk   abundant;    segmentation   dis- 
coidal;  amnion  and  allantois  present. 

421.  Form. — The  birds,  like  many  of  the  extinct  reptiles, 
are  bipeds.  The  axis  of  the  more  or  less  stout  body  makes  an 
angle  of  varying  size  with  the  axis  of  the  legs,  that  is,  the 
vertical.  The  sacrum  and  the  soft  parts  of  the  body  project 
behind  this  point  of  union  in  such  a  way  as  to  balance  the 
anterior  parts.  The  anterior  appendages  are  not  always  well 
developed  but  are  much  anterior  to  and  above  the  centre  of 
gravity.  This  results  in  a  more  stable  position  of  the  body 
in  flight.  The  posterior  appendages  are  relatively  long,  some- 
times extraordinarily  so.  In  all  cases  there  is  an  interesting 
correlation  between  the  length  of  the  neck  and  that  of  the  legs. 
The  wading  birds  are  especially  endowed  in  these  particulars. 
The  posterior  appendages  usually  have  four  digits.  These  may 
all  be  directed  forward  as  in  some  swifts,  or  much  more  com- 
monly the  great  toe  (number  i)  is  directed  backward;  in  some 
species  two  are  turned  backward  and  two  forward.  In  swim- 
ming birds  a  web  is  present  which  stretches  from  toe  to  toe. 
The  special  form  and  arrangement  of  the  web  differ  in  dif- 
ferent species.  The  digits  end  in  claws  which  vary  greatly  in 
accordance  with  the  habits  of  the  possessor.  The  anterior 
appendages  usually  show  traces  of  three  much  reduced  digits. 

422.  Supplementary  Studies. — Allow  students  to  make  a  series  of 
studies  of  the  angle  made  by  the  axis  of  the  body  with  a  vertical  line 
in  various  birds.  Compare  this  angle  in  the  robin  when  at  rest  and 


ZOOLOGY. 


when  running.  Make  outline  drawings  of  the  shank  and  toes  of  all  the 
types  of  birds  which  can  be  found,  and  discuss  the  differences  in  the  light 
of  the  habits  of  the  birds.  Make  figures  of  the  varieties  of  webs  found 
in  the  aquatic  birds. 

423.  Covering. — The  form  of  birds  as  outlined  above  is 
much  modified  by  the  presence  of  feathers.  They  increase  the 
stretch  of  the  wings  and  the  surface  exposed  to  the  air,  and 
thus  are  important  as  aids  to  flight.  In  addition  they  are  pro- 
tective in  several  respects.  The  pigment  possessed  by  the 
feathers  serves  to  enhance  greatly  the  beauty  and  variety  of 
the  members  of  the  group.  That  the  color  patterns  are  of 

FIG.  201. 


FIG.  20 1.     Diagram  showing  the  tracts  where  the  principal  growth  of  feathefs  occurs 
(Upupa  epops).     From  Bronn.     The  dotted  areas  are  the  pteryla. 

Questions  on  the  figure.— Is  this  a  dorsal  or  a  ventral  view?  Find  a 
figure  giving  the  opposite  view  of  some  bird  and  compare  with  this.  Is 
there  variety  in  the  different  species  of  birds  as  to  the  distribution  of  the 
growth  of  feathers? 


AVES.  399 

distinct  value  in  sexual  attraction  has  been  believed  by  many 
naturalists.  The  feathers,  together  with  the  scales  of  the 
shank,  the  claws,  and  the  beak,  are  epidermal  growths  and 
represent  the  remnants  of  the  exoskeleton  so  well  developed  in 
some  of  the  lower  forms.  Feathers  are  not  usually  produced 
uniformly  over  the  body,  but  are  grouped  in  regions  \vhich 
differ  in  different  species.  They  also  vary  a  great  deal  in  form, 
from  the  down  feathers  of  the  young  to  the  stiff  quill-feathers 
of  the  wings  and  tail.  Most  birds  shed  their  feathers  either 
a  few  at  a  time  the  year  round  or  within  a  short  period.  In 
the  former  case  the  change  may  be  scarcely  noticeable.  When 
the  moulting  takes  place  rapidly,  it  may  be  accompanied  by 
profound  disturbance  in  the  health  and  habits  of  the  animal. 
The  new  feathers  may  differ  in  color  from  the  old,  and  thus 
a  periodic  change  is  apparent  in  the  dress  of  some  of  our  birds. 
This  is  not  infrequently  of  such  character  as  to  accord  in  color 
with  the  changes  in  nature  outside,  giving  a  real  protective 
value. 

424.  Supplementary  Topics. — In  what  various  ways  are  the  feathers 
of  birds  protective?    Explain  how  the  protection  is  realized  in  each  case. 
What  varieties  of  feathers  may  be  found  in  birds,  and  what  are  the  chief 
differences  in  structure?     How  are  the  color  patterns  obtained?     Are  they 
made  up  of  feathers  of  one  color  so  put  together  as  to  form  the  pattern, 
or  is   a   single   feather  of  more   than   one   color?     Does   a   single   feather 
ever  show  an  independent  complex  color  pattern?     Where  is  the  boundary 
between   feathers  and   scales  on   the   legs  of  various  breeds   of  chickens? 
Do  you  find  any  evidences  that  feathers  are  highly  modified  scales?     Are 
any  of  the  feathers  like  the  hair  of  mammals? 

Secure  further  data  from  nature  and  from  reference  books  concerning 
the  moulting  habits  of  birds. 

425.  Endoskeleton. — The  chief  points  of  importance  to  the 
elementary  student  are  as  follows : 

i.  There  is  a  fusion  of  several  vertebrae  in  the  sacral  region 
(including  some  of  the  thoracic,  all  of  the  lumbar,  the  sacrals 
and  the  caudals)  with  the  dorsal  bones  of  the  pelvic  girdle,  to 
form  a  strong  dome-shaped  structure  above  the  viscera.  The 
cervical  vertebrae  vary  much  in  number  (eight  to  twenty- four) 
with  the  length  of  the  neck. 


4°°  •       ZOOLOGY. 

2.  The  cranial  bones  fuse  closely,  and  the  bones  of  the  face 
are  prolonged  into  the  core  for  the  beak  (Fig.  216). 

3.  The  sternum  is  normally  well  developed  and  provided 
with   a   keel   to   which   the   muscles    of    flight   are    attached. 
Finger-like  processes  also  increase  its  surface  for  the  attach- 
ment of  muscles  and  the  support  of  the  viscera. 

4.  The  ribs  are  double-headed,  and  each  has  a  process  on 
the  posterior  margin,  joining  it  to  the  rib  behind. 

5.  The  pectoral  girdle  has  its  clavicles  fused  ventrally  in  the 
flying  birds,  forming  the  "  wish  bone." 

6.  In  the  pelvic  girdle  the  ventral  bones  (ischium  and  pubis) 
both  pass  backward  from  the  hip  joint  and  support  the  viscera. 

7.  The  ankle  region  of  the  birds  is  very  characteristic.    The 
proximal  tarsals  unite  with  the  tibia,  and  the  distal  tarsals  unite 
with  the  fused  metatarsals  to  form  the  tarso-metatarsus  or 
shank.     The  joint  is  between  the  proximal  and  distal  tarsals 
(see  Fig.  159). 

426.  Digestive  Organs. — The  horny  beak  entirely  replaces 
the  teeth  in  the  modern  birds.     In  the  early  members  of  the 
group  teeth  are  known  to  have  been  present.    The  oesophagus, 
often  of  great  length,  is  usually  expanded  into  a, non-glandular 
crop,  where  the  food  is  stored  and  softened.     The  stomach 
often  consists  of  two  portions,  the  anterior  glandular  pro- 
ventriculus  and  the  posterior  muscular  gizzard.    In  birds  which 
habitually  feed  on  grains  or  other  hard  objects  the  inner  wall 
of  the  gizzard  is  lined  with  a  hard  and  thickened  cuticle  which 
assists  in  grinding  the  food.    Fragments  of  rock,  sand,  etc.,  are 
nearly  always  swallowed  by  grain-eating  forms  to  assist  in 
the  process.    These  are  manifestly  devices  to  do  work  usually 
done  by  teeth.     The  usual  glands  are  found  associated  with 
the  digestive  tract,  excepting  the  salivary.    The  tract  ends  in  a 
cloaca. 

427.  Supplementary  Studies. — What  is  the  exact  position 
of  the  crop?  Advantage  of  this  position?  Make  a  comparative 
study  of  the  beak  in  various  birds;  how  adapted  to  the  habits? 


AVES.  4OI 

Is  there  any  recorded  evidence  that  the  character  of  the  gizzard 
in  a  given  individual  may  vary  somewhat  in  accordance  with 
the  food  used? 

428.  Respiration. — The  trachea  corresponds  in  length  to 
the  length  of  the  neck.     Its  rings  are  rigid   (ossified).     It 
divides  into  a  right  and  left  bronchus  which  pass  to  the  re- 
respective  lungs.     The  lungs   are  closely  applied,   and  even 
attached,  to  the  dorsal  wall  of  the  thorax  and  are  small  in  pro- 
portion to  the  size  of  the  animal.     Some  of  the  bronchial  tubes 
connect  with  air  spaces  (nine  in  the  pigeon)  among  the  viscera 
and  extending  even  into  the  hollow  bones.    They  are  probably 
chiefly  respiratory  in  function. 

Bird  notes  are  produced  not  at  the  upper  end  of  the  trachea 
as  in  other  vertebrates  but  near  its  lower  end,  where  it  joins 
the  bronchi.  The  organ  is  called  the  syrinx.  Its  mode  of 
action  is  somewhat  similar  to  that  of  the  vocal  cords  in  the 
larynx  of  mammals. 

429.  The  Nervous  System  and  Organs  of  Special  Sense. 

— The  cerebral  hemispheres  are  relatively  larger  than  in  any 
of  the  groups  yet  studied.  Their  surface  is  smooth.  The 
cerebellum  is  also  large  and  concentrated  chiefly  in  a  central 
or  median  lobe.  By  the  growth  of  these  two  portions  the 
well-developed  optic  lobes  are  crowded  into  a  lateral  position. 
The  olfactory  lobes  are  small  and  the  sense  of  smell  is  not  so 
acute  as  in  many  other  vertebrates.  The  optic  lobes  and  the 
eyes  are  well  developed  and  the  sense  of  sight  is  correspond- 
ingly acute.  The  eye  protrudes  as  a  somewhat  rounded  cone 
in  front.  This  is  supported  by  a  ring  of  sclerotic  (bony) 
plates.  The  power  of  accommodation,  that  is,  of  focusing  the 
eye  upon  objects  at  different  distances,  is  very  great  in  birds. 
In  addition  to  the  upper  and  lower  lids  a  transparent  fold  of 
the  conjunctiva  (nictitating  membrane)  may  be  drawn  over 
the  eye  from  the  inner  corner.  Hearing  is  acute,  and  the  con- 
dition of  the  ear  is  interesting  chiefly  in  the  facts  of  the  ab- 
sence of  the  concha  of  the  external  ear,  and  in  the  presence 
of  a  well-developed  but  uncoiled  cochlea  in  the  internal  ear. 
27 


402 


ZOOLOGY. 


430.  Habits. — None  of  the  animal  groups  present  habits 
more  interesting,  more  readily  studied  or  more  suggestive  of 
the  adaptation  of  structure  to  the  demands  of  the  environment 
than  do  the  birds.  The  student  will  find  by  observation  and 


-od 


Xu.o°'°- 


FIG.  202.  Diagram  of  the  female  genital  organs  of  a  Bird,  c,  cloaca;  i,  intestine; 
k,  kidney;  o,  ovary  with  ova  of  different  size;  od.,  oviduct;  o.f.,  funnel  of  the  oviduct; 
0.0.,  opening  of  the  oviduct  into  the  cloaca;  u,  ureter;  u.o.,  opening  of  ureter  into  the 
cloaca.  Only  one  ovary  and  oviduct  are  fully  developed  in  the  Birds. 

Questions  on  the  figure. — What  openings  has  the  oviduct  ?  Why  must 
the  union  of  sperm  and  ovum  take  place  before  the  egg  gets  well  down 
the  oviduct?  Define  the  cloaca.  On  which  side  are  the  sexual  organs 
rudimentary  in  the  female  bird? 

FIG.  203.  Diagram  of  the  male  urino-genital  organs  of  a  Bird,  ad.,  adrenal  body; 
c,  cloaca;  i,  intestine;  k,  kidney;  t,  testis;  u,  ureter;  u.o.,  opening  of  ureter  into  the 
cloaca;  v.d.,  vas  deferens;  v.d.o.,  opening  of  the  vas  deferens;  r.s.,  vesicula  seminalis. 

Questions  on  the  figure. — What  is  the  function  of  the  vas  deferens? 
Of  the  vesicula  seminalis?  What  differentiates  the  cloaca  from  the  in- 
testine? What  are  the  chief  differences  in  the  excretory  organs  of  birds 
and  mammals? 


AVES.  403 

by  reference  to  current  works  on  natural  history  many  inter- 
esting facts  in  connection  with  bird-life.  Under  the  sugges- 
tive studies  a  partial  list  of  such  topics  will  be  found.  In  the 
chapter  on  Adaptations  and  in  the  section  on  the  classification 
of  birds  (Ch.  VIII,  and  §  432)  additional  facts  have  been  pre- 
sented. Much  of  the  time  given  to  the  practical  studies  of  the 
group  of  birds  should  be  directed  to  their  life  and  adaptations. 
These  various  habits  and  modes  of  life  have  frequently  been 
made  the  basis  of  classification :  for  example,  some  fly  and 
some  do  not ;  some  wade,  having  long  legs ;  others  swim  and 
have  webbed  feet;  some  capture  living  prey  with  talons  and 
curved  beak ;  some  scratch  and  have  blunted  claws ;  some  climb 
and  have  two  digits  directed  forward  and  two  backward; 
others  perch  and  have  only  one  toe  pointed  backward.  The 
resort  to  such  superficial  features  in  classifying  birds  suggests 
that  the  members  of  the  class  are  more  nearly  related  and  more 
similar  among  themselves  in  the  fundamental  features  of  struc- 
ture than  is  the  case  with  the  subdivisions  of  the  other  classes 
of  vertebrates. 

431.  Reproduction  and  Development. — Reference  has 
already  been  made  to  the  fact  that  the  right  reproductive 
organs  of  the  female  birds  are  much  reduced  or  wanting. 
The  ovum  is  always  large,  containing  abundant  yolk.  When 
mature  it  breaks  from  the  ovary,  enters  the  funnel-shaped  end 
of  the  oviduct  and  as  it  passes  outward  is  fertilized.  It  then 
receives  a  layer  of  albumen,  and  later  is  surrounded  by  a  mem- 
branous covering  and  by  a  porous,  limy  shell,  all  of  which  are 
secreted  by  the  walls  of  the  oviduct.  The  protoplasm  is  con- 
fined to  a  small  germinal  disc  and  segmentation  is  discoidal, 
resulting  in  a  blastoderm  like  that  of  reptiles.  In  the  newly 
laid  egg  cleavage  is  well  advanced.  After  the  egg  is  laid, 
cleavage  is  checked  until  the  necessary  temperature  for  further 
development  is  supplied  either  by  the  brooding  of  the  parent 
or  by  some  special  device.  Owing  to  the  action  of  gravity 
on  the  heavier  yolk  the  living  disc  is  always  directed  upward, — 
the  position  most  favorable  for  getting  the  warmth  of  the 


4°4 


ZOOLOGY. 


mother's  body  in  incubation.  For  the  details  of  further 
development  the  student  must  be  referred  to  more  extensive 
texts,  but  it  may  be  stated  that  the  blastoderm  comes  to  consist 
of  two  layers  of  cells  which  have  been  likened  to  two  watch 
glasses  so  placed  as  to  enclose  a  shallow  cavity.  The  outer 

FIG.  204. 


FIG.  204.  Diagram  of  a  longitudinal  section  of  the  embryo  of  a  fowl,  without  the 
amnion  and  allantois.  Ectodermal  boundaries  are  in  continuous  lines,  the  entodermal 
and  mesodermal  are  in  broken  lines:  the  entodermal  of  short  dashes,  the  mesodermal  of 
long,  b,  brain;  b.w,,  body  wali>  c.c.,  central  canal  of  spinal  cord;  co.,  ccelom;  g,  gut; 
g.w.,  wall  of  gut:  s.c.,  spinal  cord;  y.s.,  yolk  sac. 

Questions  on  the  figure. — What  is  the  relation  of  the  yolk  sac  to  the 
digestive  cavity?  Which  of  the  embryonic  layers  surrounds  it?  In  what 
way  is  the  abundant  yolk  in  the  yolk  sac  brought  into  the  circulation  of  the 
embryo  (see  reference  texts)  ? 

layer  is  ectodermal  and  is  continuous  at  the  edge  with  the 
inner,  which  is  composed  of  larger  cells  incompletely  separated 
from  the  yolk  beneath  (Fig.  1 1,  C,  4)'.  This  inner  layer  gives 
rise  to  both  entoderm  and  mesoderm.  The  blastoderm  contin- 
ues to  grow  at  the  margins  until  the  yolk  is  entirely  enveloped 
by  a  living  membrane  which  is  well  supplied  with  blood  vessels 


AVES. 


4°5 


and  serves  to  extract  the  food  for  the  use  of  the  embryo  and 
to  aerate  the  blood  before  the  lungs  become  of  use.  The  am- 
nion  and  allantois  (see  §  415 ;  Fig.  205)  are  both  developed  as 
in  reptiles.  Of  these  the  amnion  appears  first.  By  a  study  of 
Figs.  204  and  205,  together  with  others  in  the  reference  texts, 
it  will  be  seen  that  the  amnion  is  an  outgrowth  of  the  body- 
wall  of  the  embryo  and  has  a  cavity  continuous  with  the 
ccelom.  The  outer  layer  is  known  as  the  false  amnion;  the 
inner  is  the  true  amnion  (Fig.  204,  am2,  am1).  Into  the  space 
between  the  amnion-layers  the  wall  of  the  gut  outpockets, 


FIG.  205. 


am.c 


am 


CO. 


al 


FIG.  205.  Diagram  of  a  longitudinal  section  through  the  embryo  of  a  fowl,  show- 
ing formation  of  amnion  and  allantois  and  the  relation  of  these  membranes  to  the 
embryo.  The  boundaries  are  as  in  the  preceding  figure,  am1,  inner  or  true  amnion; 
am2,  outer  or  false  amnion;  am.c.,  amniotic  cavity;  al.,  allantois;  c.c.,  central  canal  of 
the  spinal  cord;  co.,  ccelom;  g,  gut;  ys.,  yolk  sac. 

Questions  on  the  figure. — Which  of  the  three  embryonic  layers  enter 
into  the  amniotic  folds?  Which  go  to  form  the  allantois?  Show  that 
the  cavity  between  the  true  and  false  amnion  is  "  extra-embryonic  "  ccelom. 
How  is  the  amniotic  cavity  lined?  With  what  is  the  yolk  sac  lined?  The 
cavity  of  the  allantois  is  in  reality  a  portion  of  what  cavity?  Which  of 
these  membranes  unite  in  mammals  to  form  the  chorion  (see  §450- 


406 


ZOOLOGY. 


forming  the  allantois.  The  cavity  of  this  sac  is  continuous 
with  the  lumen  of  the  gut.  The  embryo  thus  becomes  com- 
pletely surrounded  by  protective  membranes.  The  cavity 
between  the  true  amnion  and  the  body  wrall  (Fig.  205,  am.  c,) 
is  the  amniotic  cavity  and  may  be  filled  with  a  fluid. 

FIG.  206. 


L'IG.  200.     Archecoptcry.r  lithographica,   an  early  Reptilian   Bird.      From   Claus. 

Questions  on  the  figure. — What  in  the  figure  shows  this  to  be  a  bird? 
What  shows  it  to  be  different  from  typical  birds?  What  is  signified  by 
each  of  the  terms  in  its  scientific  name? 


FIG.  207. 


FIG.  207.     Apteryx  anstralis.     From    Romanes. 

Questions  on  the  figure. — What  peculiarities  does  this  bird  present? 
What  does  Apteryx  mean?  What  is  the  distribution  of  this  species? 
What  are  its  nearest  relatives  among  the  birds? 


AVES. 


407 


432.  Classification  of  Aves. 

Subclass  I.  Saiirnrce  (Tailed,  Reptilian  Birds). — These  are 
extinct  birds  related  to  the  extinct  reptiles — the  dinosaurs — in 
having  a  vertebrated  tail,  and  jaws  bearing  teeth.  Each  ver- 
tebra of  the  tail  possessed  a  pair  of  feathers,  the  tail  thus  hav- 
ing a  row  of  rectrices  on  either  side. 

FIG.  208. 


FIG.  208.     Ostrich    (Struthio).     From   Wood's   Natural    History. 

Questions  on  the  figure. — Which  of  the  types  of  feathers  of  ordi- 
nary birds  become  the  plumes  in  the  ostrich?  What  is  the  real  size  of 
the  ostrich  ? 

Archgeopteryx,  of  which  two  specimens  have  been  found  in 
the  lithographic  quarries  of  Bavaria,  represents  the  group  and 
was  about  the  size  of  a  crow  (Fig.  206). 

Subclass  II.  Neornithes  (Modern  Birds). — This  group  is 
characterized  by  the  reduction  and  fusion  of  the  tail  vertebrae 


408  ZOOLOGY. 

in  such  a  way  that  the  tail  feathers  (rectrices)  are  arranged 
in  a  semi-circle  (or  sometimes  wanting).  Teeth  are  wanting 
except  in  some  extinct  forms,  which  stand  intermediate  be- 
tween the  Saururae  and  the  recent  birds. 

Division  I.  Ratitce. — These  are  running  birds  with  a  flat 
breast  bone  (i.  e.,  no  keel)  and  with  all  the  organs  of  flight 
much  reduced.  The  barbs  of  the  feathers  are  not  held  to- 
gether by  barbules,  thus  producing  plumes. 

FIG.  209. 


FIG.  209.     Wood  Duck   (Aix  sponsa).     Photographed  by  Dr.  J.  W.   Folsom. 

The  Ratitae  (order  Cursor -es)  are  the  lowest  forms  of  living 
birds  and  include  the  ostriches,  emus,  cassowaries,  in  all  of 
which  the  wings  are  reduced,  and  the  Apteryx  or  wingless 
bird  of  New  Zealand  (Fig.  207)  in  which  they  are  very  rudi- 
mentary. The  ostrich  (Fig.  208)  is  the  largest  and  most  pow- 
erful of  living  birds.  Ostriches  are  somewhat  gregarious,  and 
frequent  regions  more  or  less  desert.  At  mating  time  they 
unite  in  pairs,  the  male  assisting  in  incubating  the  eggs,  which 


AVES. 
FIG.  210. 


4°9 


FIG.  210.  Ross'  Gull  (Rhodostethia  rosea).  Upper  figure  adult  male;  lower,  young 
female.  From  "  Chapters  on  Natural  History  " ;  drawn  by  Dr.  R.  W.  Shufeldt  after 
Ridgway. 

Questions  on  the  figure. — What  indications  of  structural  adaptation 
to  habits  do  you  find  in  the  figure  ?  What  sexual  dimorphism  is  per- 
ceptible? 

are  laid  in  holes  in  the  sand.  Ostrich  culture  is  an  important 
industry  in  South  Africa  and  to  a  certain  extent  in  America, 
on  account  of  the  plumes  which  are  extensively  used  as  orna- 
ments. Besides  the  types  mentioned  there  are  a  number  of 
extinct  forms  belonging  to  this  division,  some  of  which  have 
become  extinct  in  recent  time.  ZEpyornis  is  one  of  these, 
formerly  a  native  of  Madagascar,  where  remnants  of  its  eggs 
have  been  discovered  showing  that  its  volume  was  about  six 
times  that  of  the  ostrich  egg,  i.  e.,  having  a  capacity  of  about 
two  gallons. 


410 


ZOOLOGY. 


Division  II.  Carinatcc. — Birds  with  the  keeled  breast  bone, 
the  wings,  and  the  other  organs  of  flight  usually  well  devel- 
oped. Barbs  of  the  feathers  have  barbules.  All  the  modern 
flying  birds  are  embraced  in  this  subclass. 

The  further  subdivisions  of  the  Carinatse,  as  given  in  the 
recent  classifications,  based  upon  internal  structure,  are  entirely 
unsuited  for  beginners.  An  older  arrangement  of  the  prin- 
cipal orders,  based  upon  habits  and  certain  superficial  features, 
is  presented  below  for  the  convenience  of  the  student.  It 
should  be  remembered  however  that  the  classification  is  not 
the  best  possible,  inasmuch  as  forms  in  reality  not  very  closely 
related  in  structure  are,  according  to  it,  placed  together  because 
of  similar  habits. 

FIG.  211. 


FIG.  211.     Green  Heron  (Ardea  virescens).     Photographed  by  Dr.  R.  W.  Shufeldt. 

Questions  on  the  figure. — To  what  order  of  birds  does  the  heron  be- 
long? What  are  its  nearest  relatives?  What  can  you  say  of  the  habits 
of  the  order? 


AVES.  411 

The  order  Natatorcs  includes  the  divers  and  swimmers,  as 
the  auks,  penguins,  petrels,  gulls,  the  albatross,  ducks  and 
geese.  The  legs  are  usually  short,  and  the  toes  are  webbed. 
The  auks  and  penguins  have  poorly-developed  wings  and  are 
almost  helpless  on  shore.  At  the  other  extreme  are  the  power- 
ful fliers,  the  gulls,  petrels,  wild  geese  and  the  like.  Many  of 


FIG.  212.     A  right  lateral  view  of  the  skull  of  the  American  Flamingo   (Phcenicopterus 
ruber).     Photographed  from  specimen  by  Dr.   R.  W.   Shufeldt. 

Questions  on  the  figure. — Distinguish  upper  and  lower  jaws,  com- 
paring them  as  to  massiveness.  Is  this  the  usual  condition  in  birds?  How 
much  of  the  skull  is  occupied  by  the  brain?  To  what  habits  of  the 
flamingo  is  the  form  of  its  beak  an  adaptation?  Compare  with  Fig.  216. 

these  dive  in  capturing  prey  or  avoiding  enemies,  and  some 
have  the  power  of  swimming  under  water  for  considerable 
distances  (Figs.  209,  210). 

The  Grallatores,  or  waders,  have  relatively  long  legs,  neck 
and  beak.  The  toes  are  often  partly  webbed.  The  food  con- 
sists of  small  water  animals.  Here  are  included  the  game- 


4I2 


ZOOLOGY. 
FIG.  213. 


FIG.  213.     Pelican  (Pelecanus  erythrorhynchus).     By  Folsom. 

Questions  on  the  figure. — What  is  the  nature  and  purpose  of  the  fold 
beneath  the  jaw?    To  what  division  of  the  birds  does  the  pelican  belong? 

birds,  the  snipe  and  plover ;  the  egret,  almost  exterminated  in 
furnishing  feathers  for  human  adornment ;  the  cranes,  storks, 
herons,  rails  and  the  bittern.  These  birds  frequent  marshy 
Chores  and  shallow  streams,  where  their  food  abounds  (Figs. 
211-213). 

The  Gallince  comprise  a  number  of  well-known  land  birds. 


AVES.  413 

some  of  which  have  proven  very  useful  to  man.  The  barn- 
yard fowl,  turkey,  guinea-fowl,  pea-fowl,  pheasant,  grouse  and 
quail,  are  good  examples.  They  are  characterized  by  strong 
legs  with  nails  flat  and  suited  to  scratching.  The  beak  is  stout 
and  bent  downward  at  the  point.  The  feet  are  adapted  to 

FIG.  214. 


FIG.  214.     Ruffed  Grouse  (Bonasa  umbellus).     Photographed  by  J.  W.  Folsom. 

perching.  As  a  rule  they  are  poor  flyers.  It  will  be  seen 
that  the  order  furnishes  many  of  the  game  birds  of  the  world, 
and  under  proper  restrictions  the  wild  species  may  be  made  to 
contribute  materially  to  the  food  supply.  As  is  true  in  many 
other  cases,  however,  the  very  presence  of  man  in  large  num- 
bers makes  extermination  or  domestication  the  only  alterna- 
tives. Chief  among  all  the  domesticated  birds  is  the  common 
fowl,  which  is  descended  from  a  species  native  to  southern 
Asia  (Gallus  bankiva).  The  varieties  resulting  from  human 


ZOOLOGY. 


AVfcS. 


415 


selection  are  remarkable  in  the  extreme.  The  egg  and  fowl 
industry  is  one  of  the  most  important  in  the  country,  amount- 
ing according  to  estimates  to  more  than  400,000,000  dollars 
annually.  In  1901,  one  and  one-half  billion  dozens  of  eggs 
were  produced  on  the  farms  of  this  country.  It  is  readily  seen 
that  this  is  one  of  the  large  sources  of  food  for  man  (Figs. 

214-216). 

FIG.  216. 


FIG.  216.     Skulls-  of  gallinaceous  Birds,  as  Partridge,  Grouse,  etc.     Photographed  from 
the  specimens  by  Dr.   R.  W.  Shufeldt.     Adult.     J  natural  size. 

Questions  on  the  figures. — Compare  these  skulls  and  note  the  points  of 
similarity  and  dissimilarity.  Find  the  position  of  eye,  ear,  and  nares. 
What  are  the  chief  points  of  contrast  between  these  skulls  and  that  of  the 
owl  (Fig.  219),  and  of  the  flamingo  (Fig.  212)? 


416 


ZOOLOGY. 


The  Columbcc  embrace  birds  somewhat  similar  but  with 
weaker  legs  than  those  of  the  preceding  order.  They  have 
straight  beak  and  compressed  nails.  The  pigeons,  doves,  and 
the  recently  extinct  dodo  are  types.  The  domestic  pigeons,  of 
which  there  are  many  interesting  varieties,  are  the  descendants 

FIG.  217. 


FIG.   217.     Great   horned-owl    {Bubo   virginianus).     Adult   female.     Photographed   from 
life  by  Dr.    R.   W.   Shufeldt. 

Questions  on  the  figure. — What  are  the  habits  of  owls?  Does  the 
figure  show  any  structural  adaptations  to  known  habits?  How  does  the 
standing  position  of  the  owl  differ  from  that  of  other  birds  of  your 
acquaintance?  See  also  Figs.  218  and  219. 


AVES. 

FIG.  218. 


417 


FIG.   2i'8.     Great  horned-owl    (Bubo  virginianus),   Young.     Photographed   from  life  by 

Dr.  R.  W.  Shufeldt. 

Questions  on  the  figure. — Compare  the  young  at  all  points  with  the 
adnlt.     What  are  the  points  of  difference?     Of  manifest  likeness? 

of  the  rock-pigeon  (Columba  livia]  of  southern  Europe.  The 
differences  in  form  and  structure  between  the  different  artifi- 
cial varieties  of  pigeon  are  much  greater  than  are  found  sep- 
arating many  natural  species  or  even  genera. 

The  order  Raptores  includes  the  birds  of  prey,  as  the  eagles, 
vultures,   hawks,   owls.     They   are   characterized   by   hooked 
beak,  strong,  curved  claws,  and  great  powers  of  flight.     The 
28 


4*8  ZOOLOGY. 

vultures  and  their  allies  are  scavengers  and  are  thus  a  distinct 
advantage  to  man.  The  remainder  prey  upon  living  animals, 
chiefly  vertebrates  (Figs.  217-219). 

FIG.  219. 


FIG.  219.  Skull  of  Owl  (Syrnium  nebulosum).  After  Shufeldt,  photographed  from 
specimens.  Upper  figure  bisected,  showing  brain-case;  the  lower  from  a  dorsal 
aspect. 

Questions  on  the  figure.— Is  the  owl  a  bird  of  prey?  What  is  the 
position  of  the  eyes  in  relation  to  the  skull?  Of  the  nares?  Compare 
these  figures  with  the  head  of  the  owl  (Fig.  217),  and  with  the  skulls 
in  Fig.  216. 

The  owls  have  interesting  habits  in  that  they  are  nocturnal 
flyers,  and  during  the  day  retire  to  dark  places,  where  they 
sit  quietly.  They  thus  reverse  the  habits  of  the  majority  of  ver- 
tebrates in  the  use  of  day  and  night.  While  there  are  many 
nocturnal  vertebrates,  there  are  few  so  helpless  in  the  light  as 
the  owls.  Their  prey,  in  many  instances,  are  nocturnal  animals. 


AVES. 


419 


The  Picarice,  or  woodpecker-like  birds,  have  two  toes  in 
front  and  two  directed  backward  in  adaptation  to  their  climb- 
ing habit.  The  beak  is  usually  strong,  but  varied  in  shape. 
The  woodpeckers,  cuckoos,  the  toucans  or  hornbills  and  the 
kingfishers  may  be  included  here.  Related  to  this  order  are 

FIG.  220. 


FIG.   220.     Belted  Kingfisher    (Ceryle  alcyon,  L.).     About  one-fourth  natural  size. 
By  J.  W.  Folsom. 

the  parrots.  Some  species  of  cuckoo  lay  their  eggs  in  the 
nests  of  other  birds,  where  they  are  hatched  and  nourished  by 
the  host,  and  are  said  to  cast  the  proper  owners  from  the  nest 
in  return  for  the  care  they  receive.  The  woodpeckers  drill 
holes  in  the  bark  and  in  the  wood  of  decaying  trees  in  search 
of  the  insects  frequenting  such  places.  Their  nests  are  exca- 
vated by  them  in  a  similar  way.  The  same  pair  may  use  the 
same  nest  many  years  in  succession  unless  forestalled  by  indi- 
viduals of  some  more  war-like  species  which  refuse  to  yield  it. 
The  tapping  sounds  made  by  the  woodpecker  serve  often  rather 


420 


ZOOLOGY 
FIG.  221. 


FIG.  221.     Arctic    three-toed    Woodpecker.     From    U.    S.    Dept.    Agriculture,    "  North 

American  Fauna." 


FlG.    222. 


FIG.  222.     Yellow-billed    Cuckoo    (Coccyzits    americanus) .     Adult    male.     Photographed 
from  life  by  Dr.  R.  W.  Shufeldt. 

Question  on  the  figure. — What  are  the  nearest  relatives  of  the  cuckoos 
among  the  birds? 


FIG.  223.     Clark's    Crow.     U.    S.    Dept.    Agriculture:    "  North    American    Fauna.' 


FIG.  224. 


FIG.  224.     Nestling  Crows   (.Corvus).     From  U.  S.   Dept.  Agriculture  Year-book,    1900. 


422 


ZOOLOGY. 


to  frighten  the  insects  and  drive  them  from  cover  than  in 
actual  excavation.  In  some  species  the  tapping  is  a  means  of 
attracting  the  mates.  In  some  cases  these  birds  bury  acorns 
in  holes  which  they  have  made  in  the  bark  of  trees,  returning 
for  them  when  other  food  supplies  are  low.  Somewhat  inter- 
mediate between  this  group  and  the  next  are  the  humming 
birds,  the  chimney  swifts  and  the  whippoorwill. 

FIG.  225. 


,  &MsL&j/fc.  "> 


£>, 


FIG.  225.     Gold-finch    (Spinus  tristis).     U.    S.    Dept.   Agriculture   Year-book,    1898. 

The  Passeres  is  a  very  large  order,  embracing  numerous 
families  of  birds,  mostly  small,  with  three  toes  in  front  and 
one  behind,  and  adapted  to  perching.  The  majority  are 
gifted  with  some  powers  of  song  (O seines).  A  few,  of  which 
the  king  bird  may  stand  as  the  type,  are  known  as  crying  birds 
(Clamatores).  The  Passeres  include  probably  one-half  the 
species  of  birds.  Here  are  found  the  sparrows,  the  thrushes, 
the  wrens,  the  larks,  the  swallows,  the  crows,  and  their  allies 
(Figs.  223-229). 


423 


FIG.  226.     Mocking   Bird    (Mimus  polyglottos).     From   Dept.   Agriculture 
Year-book,   1895. 


FIG.  227. 


FIG.  227.     Wood  Thrush   (.Hylocichla  mustelina).     Female,   one-half  size.     By 
J.  W.  Folsom. 


424 


ZOOLOGY. 


For  further  description  of  the  numerous  interesting  families 
of  Passeres  the  student  must  refer  to  some  special  book  on 
birds.  The  study  of  their  habits  and  form  constitutes  one  of 
the  most  popular  and  entertaining  subjects  of  natural  history 
for  the  recreation  studies  of  busy  people.  Much  good,  and 
some  very  indifferent,  literature  intended  for  guidance  in  such 
studies  is  now  being  produced. 

FIG.  228. 


FIG.  228.     The    Meadow    Lark.     From    U.    S.    Dept.    Agriculture   Year-book,    1895. 

433-  Special  Topics  for  Investigation  in  Field  and  Library. 

1.  Enumerate  the  special  structural  features  which  seem  to  fit  birds  for 
successful  flight.     Compare  different  birds  as  to  these  features?     What  are 
the   different   modes   of   flight?     Compare   the   flight   of   the  buzzard,   the 
wood-pecker,  the  quail.     What  is  the  action  of  the  wings  in  flying?     Of 
the  tail?     What  is  the  effect  of  clipping  one  wing?     Why?     The  rate  of 
flight  in  different  species  of  birds. 

2.  Study  the  group   of  birds   from   the   point   of   view   of  their   social 
and  gregarious  instincts.     Are  any  solitary?    Do  any  have  varying  social 
habits  during  different  seasons? 

3.  Make  a  general  study  of  the  migrations  of  birds,  collecting  the  facts 
as   to    range,    time,    supposed    causes,    the    effects   on    the    species    and   its 


AVES. 


425 


geographical  distribution,  the  degree  of  exactness  in  routes  and  the  place 
of  return. 

4.  Make  a  special  study  of  the  birds  of  the  locality  in  which  you  are. 
Are  there  permanent  residents?  Summer  residents?  Winter  residents? 
Migrants  (those  which  stop  only  for  a  short  time  in  the  spring  or  autumn 
as  they  pass  from  south  to  north  or  the  reverse)  ?  Keep  a  record  from 
year  to  year  of  the  earliest  dates  at  which  migrating  species  are  seen 
in  your  locality. 

FIG.  229. 


FIG.  229.     Loggerhead    Shrikes     (Lanius    ludovicianus) .     By    J.    W.    Folsom. 

5.  What   diversity   is   there   in   the  mating  habits   of  birds?     Are   any 
monogamous'?     Polygamous?     What  are  the  mating  habits  of  the  cuckoo? 

6.  Make  a  report  as  to, the  nest-building  habits  of  selected  species  of 
birds.     How   do   the   nests   differ   in   location,    in   mode   of    formation,    in 
perfection?     Is  there  any  relation  between  the  character  of  the  nest  and 
the  degree  of  development  of  the  young  when  hatched?    What  range  of 
variation  in  the  number  of  eggs?     In  the  mode  of  incubation?     The  period 
of  incubation?     Care  of  the  young  after  hatching? 

7.  Compare  the  vocal  powers  of  birds  with  that  of  vertebrates  hitherto 
studied.     Compare  various  types  of  birds  as  to  the  range  and  character  of 
their  notes.     How  are  the  notes  of  birds  related  to  their  states  of  mind? 


426  ZOOLOGY. 

Which  are  more  vocal,  the  males  or  the  females  ?     What  explanations  are 
offered  for  this? 

8.  What  is  the  history  of  the  English  sparrow  in  this  country?     What 
are  its  habits  ?     How  do  you  account  for  its  rapid  spread  ? 

9.  Make  a  special  study  of  the  local  distribution  of  the  species  of  birds 
known    to    occur    in    your    vicinity.     Which    prefer    the    meadows?     The 
marshes  ?     The  streams  ?     The  woodlands  ?     Do  the  different  species  nest 
in  the  same  regions  in  which  they  feed? 

10.  The  relation  of  selected  species  of  birds  to  man.     Are  they  helpful 
or  harmful  to  his  interests?     Has  he  been  helpful  or  harmful  to  them? 


CHAPTER    XXIV. 
CLASS  V.— MAMMALIA  (MAMMALS). 

434.  Laboratory  and  Field  Work. — Almost  any  of  the 
smaller  mammals  may  be  used  in  the  following  exercise.  Dif- 
ferent species  may  be  taken  with  profit  by  the  various  mem- 
bers of  a  class.  The  chief  points  to  be  emphasized  are  the 
habits,  instincts  and  external  structure. 

I.  Habits  and  Instincts. 

What  are  its  natural  haunts?  What  explanation  can  you 
offer  therefor  ? 

How  does  it  protect  itself  from  its  enemies?  What  are  its 
enemies  ?  Is  it  active  at  night  or  by  day  ?  Reasons  ? 

What  are  its  habits  as  regards  food  ?  Evidences  ? 

What  can  you  say  of  its  power  and  manner  of  locomotion? 
Does  the  manner  of  motion  differ  materially  with  differ- 
ence of  rate? 

Social  habits?  Mating  habits?  Care  of  young  and  their 
condition  at  birth? 

Is  it  scarce  or  abundant  ?   Apparent  reasons  ? 

What  are  its  relations  to  human  interests  ? 

II.  General  Form  and  Structure. 

Identify  the  regions  of  the  body  and  compare  the  condition 
found  here  with  that  seen  in  the  birds.  Relation  of  axis 
of  body  to  appendages.  Compare  the  anterior  and  pos- 
terior appendages  at  all  possible  points,  and  show  to 
what  extent  the  work  done  by  each  is  indicated  by  the 
structure.  Examine  the- claws  and  the  soles  of  the  feet. 

Examine  the  body-covering,  and  compare  the  various  parts 
as  to  color,  character  of  hair,  etc.  Does  the  hair  com- 
pletely cover  the  body?  What  is  the  position  and  use  of 
"  whiskers  "  ?  Evidences  for  your  conclusion. 

Locate  all  the  external  openings.  Study  the  mouth  with  its 

427 


ZOOLOGY. 

contained  structures;  the  eyes:  position,  color,  lids  (is 
there  a  nictitating  membrane?)  ;  ears.  To  what  extent  is 
the  external  ear  developed? 

435.  The  Mammalia  embrace,  on  the  whole,  the  most  highly 
developed  vertebrates.    To  this  group  man  belongs.    The  birds 
are  more  highly  specialized  in  some  respects,  but  the  mammals 
surpass  the  birds  in  the  size  and  convolutions  of  the  brain,  and 
in  the  closer  relations  between  the  mother  and  offspring  both 
before  and  after  birth.    The  form  of  parental  care  seen  in  the 
Mammalia  is  an  adaptation  resulting  in  great  advantage  to  the 
young,  and  has  also  produced  a  great  improvement  in  the 
mental  qualities  of  the  parents.     The  class  contains  forms  of 
very  varying  appearance  and  perfection  of  development,  and 
suited  to  almost  every  mode  of  life.     Many  are  aquatic,  in- 
cluding the  largest  living  animals,  the  whales ;  some  burrow  in 
the  soil,  as  the  mole  and  many  rodents;  some  live  largely  in 
trees,  as  the  monkeys,  squirrels,  sloths,  etc. ;  a  very  few,  as 
the  bats,  have  acquired  the  power  of  flight;  others — the  vast 
majority — live  on  the  dry  land. 

436.  General  Characteristics  of  Mammals. 

1.  Mammalia   are  air-breathing  vertebrates   in  which   the 
covering  developed  by  the  epidermis  is  hair. 

2.  In  the  female,  mammary  glands  occur  in  the  skin,  by  the 
secretions  of  which  the  young  are  nourished. 

3.  The  diaphragm,  a  muscular  partition,  completely  sepa- 
rates the  body  cavity  into  two, — an  anterior  or  thoracic  and  an 
abdominal. 

4.  With  a  few  exceptions  the  Mammalia  are  quadrupeds. 

5.  Heart  is  four-chambered;  the  temperature  of  the  blood 
not  determined  by  that  of  the  surrounding  medium ;  red  blood 
corpuscles  not  nucleated;  one  (the  left)  aortic  arch  persists. 

6.  Two  occipital  condyles. 

7.  Chiefly  viviparous   (Monotremes  are  oviparous)  ;  fcetus 
nourished   during   early   development   in   the   uterus   of   the 
mother,  often  being  closely  connected  therewith  by  a  complex 
structure  known  as  the  placenta. 


MAMMALIA.  429 

437.  General    Survey. — There    are    three    subclasses    of 
mammals  which  differ  in  mode  of  reproduction  and  in  degree 
of  development. 

1.  The  Monotremata  are  the  lowest  and  are  characterized 
by  the  fact  that  they  lay  eggs,  like  reptiles  and  birds ;  there  is  a 
cloaca  into  which  the  alimentary,  urinary,  and  genital  canals 
open ;  the  milk  glands  are  poorly  developed.    The  class  is  rep- 
resented by  the  duck-mole, — an  aquatic  form,  and  the  spiny 
ant-eater, — both  natives  of  Australia  and  neighboring  islands 
(Fig-   235). 

2.  The  Marsupialia  possess  a  marsnpium  or  pouch,  a  fold  of 
the  skin  into  which  the  prematurely  born  young  are  placed  and 
nourished  until  able  to  take  care  of  themselves.     The  period 
of  gestation  is  short  and  the  connection  between  the  embryo 
and  the  wall  of  the  uterus  is  slight.    In  the  group  are  embraced 
the  kangaroo  and  other  Australian  forms,  and  the  opossums  of 
America.    It  is  an  interesting  fact  that  the  native  Australasian 
mammalia  all  belong  to  these  two  lower  classes    (see  Figs. 
50,  60). 

3.  In  the  Placentalia  there  is  a  placenta  or  mass  of  closely 
interwoven  maternal  and  embryonic  tissue  which  unites  the 
fcetus  with  the  wall  of  the  uterus,  by  which  arrangement  the 
young  gets  its  food  and  oxygen  from  the  blood  of  the  mother. 
The  young  are  retained  much  longer  in  the  uterus,  and  are 
consequently  much  more  mature  when  born.    All  the  common 
mammals  belong  to  this  group,  which  is  distributed  over  the 
habitable  part  of  the  earth. 

438.  Form. — The  axis  of  the  body  is  usually  separable  into 
head,  neck,  trunk,  and  tail, — though  the  last  may  be  reduced 
to  a  very  small  number  of  segments.    The  proportions  of  these 
parts  of  course  differ  much  and  are  to  be  connected  with  the 
habits  of  life.    Most  of  the  Mammalia  are  quadrupeds  (except 
the  allies  of  the  whales,  the  porpoises  and  the- sea-cows)  ;  and 
all  except  man,  and  some  of  apes  most  like  him,  have  the  axis 
of  the  body  in  a  horizontal  position  supported  by  all  four 
appendages,  or  by  the  medium.     The  aquatic  forms — whales, 


430 


ZOOLOGY. 


porpoises,  etc.,  become  more  or  less  fish-like  in  form,  in  adapta- 
tion to  the  medium.  There  is  an  enormous  range  in  size  in 
the  group, — from  the  mice  to  elephants  and  whales. 

439.  Supplementary  Topics  for  Laboratory  and  Field  Work. — Com- 
pare the  relative  size,  length,  etc.,  of  head,  neck,  trunk  and  tail  of  various 
types  of  mammals, — using  well  known  animals  and  the  figures  and  de- 
scriptions of  less  familiar  ones.  Can  you  find  any  signs  of  connection 
between  any  of  these  facts  and  known  habits  of  the  animals  studied? 


FIG.  .230.     Diagram    of    Skin    in    Mammals,    by    Folsom. 

Compare  the  anterior  with  the  posterior  appendages  in  a  selected  series 
of  mammals,  keeping  in  mind  the  following  points:  size,  length,  strength, 
uses ;  the  number  and  character  of  the  digits.  Compare  similarly  the 
corresponding  (i.  e.,  anterior  with  anterior)  appendages  in  another  series. 
Keep  in  mind,  throughout,  the  adaptations  of  structure  to  the  conditions  of 
life,  method  of  locomotion,  etc. 


MAMMALIA.  431 

440.  Integument. — The    skin,    as    in    forms    already    de- 
scribed, consists  of  two  portions, — an  ectodermal  portion,  the 
epidermis,  and  the  dermis  or  true  skin  which  is  derived  from 
the  mesoderm  (Fig.  230).     Hair  is  found  in  the  young  of  all 
mammals,   though   it  may  be   wanting  in  the   adult    (as   in 
whales),  or  may  occur  only  sparsely.     Hair  is  produced  by  the 
epidermis,  but  is  nourished  by  a  papilla  of  dermal  tissue  (Fig. 
230,  /) .    Each  hair  consists  of  a  central  part,  or  pith,  surrounded 
by  a  denser  cornified  portion,  the  cortex.     Hair  differs  much 
in  color  and  in  structure^ — from  the  soft  fur  of  the  seal  to 
the  quills  of  the  hedgehog  and  porcupine.     To  be  considered 
in  the  same  connection  with  hair  are  the  nails,  claws,  and  hoofs, 
the  scales  on  the  tail  of  the  rat  or  beaver,  and  the  horny  ma- 
terial of  horns. 

441.  Supplementary   Studies   for   Field  and   Library. — What   is   the 

economic  value  of  the  skins  of  mammals  ?  How  are  they  prepared  for  the 
uses  to  which  they  are  put?  What  animals  are  prized  for  their  hairy 
products  (fur,  wool,  etc.)  ?  What  special  qualities  must  the  hair  have 
to  be  useful  in  making  cloth? 

What  instances  can  you  adduce  of  advantageous  coloring  in  the  hair 
of  mammals?  What  variations  of  color  may  be  found  within  a  single 
species?  What  changes  of  color  are  possible  to  a  single  individual?  How 
are  these  changes  brought  about?  What  peculiar  qualities  have  the  quills 
of  the  porcupine? 

442.  Integumentary  Glands,  derived  from  the  epidermis, 
are  common  in  mammals.     Associated  with  the  hairs  are  the 
oil  glands.     Over  various  parts  of  the  body  are  long  tubular 
sweat  glands  buried  in  the  dermis.     The  mammary  glands, 
which  are  characteristic  of  the  group,  are  specially  developed 
skin  glands,  apparently  more  allied  to  the  oil  glands.     They 
are  much  lobed,  and  usually  have  teats  or  mammae ;  but  in  the 
monotremes  these  are  wanting,  and  the  young  merely  lick  the 
secretion  from  a  "  milk  area."    The  glands  may  be  distributed 
along  the  entire  abdominal  surface   (carnivora)   or  confined 
either  to  the  anterior  (primates)  or  posterior  portion  (rumi- 
nants).    The  number  of  the  glands  is  correlated  in  a  general 
way  with  the  number  of  young  produced  at  a  birth. 


43 2  ZOOLOGY. 

443.  Skeleton. — Some    of    the    more    elementary    facts    concerning   the 
skeleton  may  be  summarized  as  follows.     The  vertebrae  unite  by  flat  faces, 
and   the    five    regions    of   the    vertebral    column    (see    §341)    have   a    fair 
degree  of  constancy  as  to  numbers.     The  neck,   with   a   very  few  excep- 
tions, has  seven  vertebrae,  the  length  of  the  neck  depending  on  the  length 
of  the   vertebrae   and   not   on   their  number.     The   trunk   vertebrae,   made 
up  of  the  thoracic  and  lumbar,  usually  vary  within  the  limits  19-23.     The 
caudal  vertebras  are  most  variable  of  all.     The  bones  of  the  skull  in  the 
adult  have  their  edges  closely  united  by  means  of  sutures   (a  species  of 
close  joint,  which  does  not  allow  of  motion).     The  lower  jaw,  the  hyoid 
bone,  and  the  small  bones  of  the  ear  are  the  only  movable  bones  in  the 
mammalian  skull.     The  lower  jaw  articulates   directly  with  the  cranium. 
The   quadrate,    which   in   reptiles   and   birds   serves   to   articulate   the  jaw 
with  the  cranium,  has  apparently  changed  its   position  and  given  rise  to 
one  of  the  small  bones  of  the  middle  ear. 

The  pectoral  girdle  and  arm  bones  are  always  present,  but  in  the 
whales  and  sea-cows  the  posterior  are  lacking.  .The  digits  are  typically 
five  in  number.  In  many  carnivores  these  may  be  reduced  to  four, 
terminating  in  claws.  In  the  hoofed  forms  the  toes  are  often  reduced 
to  four,  two,  or  even  one  (the  horse).  In  such  cases  rudiments  of  the' 
remaining  digits  may  occur  in  the  form  of  splints. 

444.  Teeth. — The  teeth  are  produced  by  the  skin,  and  come 
to  be  lodged  in  pits  in  the  bones  of  the  jaws.    While  differing 
in  shape,  the  teeth  always  possess  the  crown,  the  fang  or  root, 
the  neck  and  the  pulp-cavity.    The  bulk  of  the  tooth  is  dentine 
deposited  by  the  dermis.    Over  this  is  a  layer  of  enamel  formed 
by  the  epidermis.    The  cavity  is  more  or  less  filled  with  "  pulp 
tissue  "  which  is  supplied  with  nerves  and  blood  vessels.    Most 
mammals  have  only  two  sets  of  teeth, — a  milk  set  which  ap- 
pears early  and  is  lost  and  a  permanent  set  which  replaces  the 
former.     In  some  cases,  however,  there  is  only  one  set,  and  in 
a  few  (e.  g.,  whales)  no  teeth  appear  above  the  surface  of  the 
gums. 

In  the  porpoises,  dolphins  and  similar  forms  the  teeth  are 
numerous,  simple,  and  very  much  alike,  but  in  the  majority 
of  mammals  there  are  at  least  three  types  of  teeth.  In  the 
front  of  the  upper  jaw  (on  the  premaxillary  bones)  are  simple, 
chisel-shaped  teeth,  the  incisors;  behind  these  (the  first  tooth 
on  the  maxilla)  is  the  canine  tooth,  usually  pointed  and  adapted 
for  tearing ;  posterior  to  the  canines  are  the  grinders  or  molars. 


MAMMALIA.  433 

Those  grinders  which  replace  milk  teeth  are  sometimes  called 
premolars.  The  true  molars  do  not  have  any  representatives 
in  the  milk  set.  The  corresponding  teeth  in  the  lower  jaw  are 
similarly  named.  The  typical  number  of  teeth  is  forty-four, 
eleven  in  each  half-jaw.  This  may  be  shown  by  a  formula  in 
which  the  numerator  indicates  the  number  of  each  kind  in  one 
half  of  the  upper  jaw  and  the  denominator  a  similar  portion 
of  the  lower :  i.  f ,  c.  y ,  p.  | ,  m.  f  —  44.  This  means  that  there 
are  three  incisors,  one  canine,  four  premolars,  and  three  molars 
in  each  half  jaw,  both  above  and  below.  The  dental  formula 
for  adult  man  is  :  i.  |,  c.  { .  p.  \  ,  m.  f  =  32.  The  numbers  are 
not  always  the  same  in  the  upper  and  lower  jaw. 

445.  Supplementary  Studies. — Let  the  student  determine  by  examina- 
tion, and  write  the  dental  formula  of  the  cat,  dog,  horse,  cow ;  milk  set 
in  man. 

Compare  the  molars  of  some  carnivorous  animals  with  those  of  some 
herbivorous;  similarly  the  canines.  Describe  the  action  of  the  jaws  in  the 
act  of  chewing  in  the  dog,  cow,  rabbit,  horse. 

446.  The  Digestive  Organs  present  the  same  regions  and 
general  arrangement  found  in  the  typical  vertebrates.     There 
are  usually  fleshy  and  movable  lips  covering  the  teeth.     Some- 


P 

FIG.  231.     Diagram  of  stomach  of  dog    (A)    and   rat    (B).     After  Wiedersheim. 

times  these  are  much  extended  and  in  connection  with  the  nose 
may  become  important  organs  (snout,  proboscis)  for  the  cap- 
ture of  food.  The  stomach  varies  widely  but  is  ordinarily  a 

29 


434 


ZOOLOGY. 


simple  sac  with  muscular  walls.  Sometimes  it  is  partly  sepa- 
rated into  chambers  by  folds  (Figs.  231,  232).  This  reaches  its 
greatest  complexity  in  the  ruminants,  in  which  four  chambers 
occur  (Fig.  232).  One  of  these — the  rumen — becomes  a  tem- 
porary receptable  for  the  food  which  is  first  swallowed  without 
being  chewed.  This  peculiar  structure  is  correlated  with  the 
habit  of  rapid  feeding  and  retirement  to  less  dangerous  or  ex- 

FIG.  232. 


FIG.  232.     Diagram  of  Stomach  of  Ruminant.     After  Wiedersheim. 

Questions  on  the  figure. — What  is  the  significance  of  the  term  rumi- 
nant? Of  what  conceivable  advantage  is  this  form  of  stomach?  What 
animals  belong  to  the  class  ? 

posed  locations,  where  the  food  is  forced  back  to  the  mouth 
in  appropriate  quantities  and  chewed  at  leisure.  When  swal- 
lowed the  second  time  the  food  passes  on  to  the  glandular 
divisions  of  the  stomach.  The  liver  and  the  pancreas  pour 
their  secretions  into  the  small  intestine  near  its  anterior  end. 
The  small  intestine  is  very  much  shorter  in  flesh-eating  ani- 
mals than  in  the  vegetable  feeders.  At  the  junction  of  the 
small  and  large  intestine  there  is  a  blind  pouch  or  sac  (ccecum, 
vermiform  appendix)  which  is  large  in  the  Herbivora,  but  in 
man  it  is  a  mere  rudiment.  It  is  doubtful  whether  it  has  any 


"    MAMMALIA. 


435 


function  in  the  human  race, 
inflammation. 


It  is  often  the  seat  of  serious 


447.  Circulatory  System. — Mammals  are  warm  blooded,  but  with  lower 
temperature  than  is  found  among  the  birds.  It  ranges  from  35°  to  40°  C. 
The  heart  is  completely  four-chambered  as  in  birds,  the  left  side  containing 
pure  blood  and  the  right  impure  (Fig.  233).  The  aorta,  arising  from  the 
left  ventricle,  has  only  one  arch — the  left,  whereas  only  the  right  is  found 

FIG.  233. 


— s 


v.c.  — 


v.c.-- 


-do, 


FIG.  233.  Diagram  of  the  heart  and  chief  vessels  in  the  mammals,  ao.,  aorta;  a.l., 
left  auricle;  a.r.,  right  auricle;  c,  carotid  artery;  d.a.,  dorsal  artery;  p.a.,  pulmonary 
artery;  p.v.,  pulmonary  vein;  s,  subclavian  artery;  v.c.,  venae  cavae  (pre-caval  and  post- 
caval);  v.l.,  left  ventricle;  v.r.,  right  ventricle. 

Questions  on  the  figure. — What  kind  of  vessels  communicate  with  the 
auricles?  What  with  the  ventricles?  What  is  the  position  of  the  valves? 
Trace  the  direction  of  the  blood  flow  in  the  various  parts  of  the  blood- 
vessels figured.  What  is  the  distribution  of  the  veins  and  arteries  shown 
here,  i.  e.,  to  what  organs  do  their  minuter  branches  go? 

in  birds.  The  general  comparison  of  the  conditions  in  vertebrates  may 
be  seen  from  the  table  on  page  340.  There  is  an  hepatic-portal,  but  no 
renal-portal,  circulation. 

The  lymphatic  vessels  are  an  important  part  of  the  circulatory  apparatus 
in  all  vertebrates.  Under  the  pressure  that  exists  in  the  arteries,  some 
of  the  fluid  portion  of  the  blood  finds  its  way  through  the  walls  of  the 
capillaries  into  the  spaces  among  the  tissues.  This  cannot  get  back  into 
the  veins,  and  hence  it  is  desirable  that  special  vessels  be  provided  to  get 
it  back  into  the  circulation.  Starting  with  the  irregular  spaces  in  the 
tissues,  in  which  the  lymph  collects,  we  find  vessels  less  regular  than  the 


43^  ZOOLOGY. 

veins,  often  running  together  and  then  rebranching,  gradually  approach- 
ing the  body  cavity.  On  their  route  they  pass  through  knots  of  special 
tissue — lymphatic  glands,  where  colorless  amoeboid  cells  are  added.  Special 
lymphatics — the  lacteals — gather  food  from  the  intestines  and,  uniting  with 
the  general  lymphatics,  finally  empty  into  the  large  veins  in  the  neck 
region.  The  escaped  lymph  is  thus  returned  to  the  blood. 

448.  The   Respiratory   Structures   differ   from   those   of 
birds  chiefly  in  the  fact  that  they  are  confined  to  the  anterior 
or  thoracic  cavity,  in  which  they  hang  freely,  suspended  by 
the  bronchi.     There  are  no  air-sacs  outside  the  lungs,  hence 
all  the  air  passages  terminate  in  the  alveoli,  in  the  walls  of 
which  are  the  pulmonary  capillaries.     Inspiration  and  expira- 
tion of  air  is  effected  by  increasing  and  decreasing  the  size  of 
the  chest  cavity  by  means  of  the  muscles  between  the  ribs  and 
by  the  contraction  of  the  muscles  of  the  diaphragm  which  is 
normally  arched  forward  into  the  chest.     By  its  contraction 
the  viscera  are  forced  backward  and  more  space  is  given  to  the 
lung,  which  at  once  fills  the  chest  cavity  as  the  result  of  air- 
pressure  on  the  inside  of  the  lung. 

449.  Nervous    System. — The    special    feature   worthy   of 
note  in  the  nervous  system  of  mammals  is  the  large  size  of  the 
brain,  especially  of  the  cerebral  hemispheres.     In  the  higher 
mammals,  particularly,  these  become  complicated  by  folds  and 
convolutions  by  which  the  surface  or  cortex  of  the  brain  is 
much  increased.    The  brain  cells,  or  gray  matter  of  the  brain, 
are  especially  abundant  in  the  superficial  part,  and  therefore 
this  increase  of  surface  means  that  these  cells  are  increased  in 
amount  as  compared  with  any  other  vertebrates.     The  intelli- 
gence of  an  animal  is  roughly  proportional  to  the  amount  of 
the  cortex.     The  fibrous  tracts  connecting,  the  various  por- 
tions  of   the   cortex   are   likewise   more  perfectly   developed 
among  the  mammals. 

The  organs  of  special  sense  are  similar  to  those  in  the  birds. 
The  ear  becomes  more  complicated.  There  is  usually  a  well- 
developed  external  ear,  or  pinna,  in  the  terrestrial  forms, 
which  is  often  movable  and  serves  to  gather  the  sound  waves. 


MAMMALIA.  437 

The  membranous  labyrinth  of  the  internal  ear  becomes  more 
complicated  than  in  any  of  the  lower  forms.  This  is  especially 
true  of  the  cochlea,  which  becomes  spirally  coiled.  The  mid- 
dle ear  is  bridged  by  a  series  of  three  bones,  instead  of  one  or 
two  as -in  the  lower  groups  of  vertebrates,  where  such  connec- 
tion exists  at  all. 

450.  The  Urinogenital  Organs. — As  in  the  other  vertebrates  there  is 
close  connection  between  the  excretory  and  reproductive  organs  in  mam- 
mals.    The  bean-shaped  kidneys  communicate  by  ureters  with   a  median 
urinary  bladder,  which  in  turn  has  the  urethra  leading  to  the  outside.    The 
urethra  also  serves  as  the  outlet  for  the  sperm  in  the  male.     The  testes, 
which    in    other   vertebrates    lie    in    the    body-cavity,    pass    backward    and 
descend   into   a   fold   of   the   skin,   in   the   majority  of  mammals.     In   the 
female,  the  ovaries  are  in  the  abdominal  cavity,  and  when  the  ova  are  ripe 
they  break  forth  into  the  cavity  and  pass  into  the  fringed,  funnel-shaped 
mouth  of  one  of  the  two  oviducts.     The  oviducts  may  be  completely  dis- 
tinct, opening  separately  into  the  vagina   (as  in  most  rodents),  in  which 
case  each  has  a  special  portion  in  which  the  young  are  retained  during 
early  development   (uterus}  ;  or  there  may  be  found  various  degrees  of 
union  of  the  uterine  portions  until  there  is  a  single  uterus  into  which  the 
two  oviducts  empty  (as  in  the  Anthropoidea). 

451.  Reproduction  and  Development. — All  the  mammals 
except  the  monotremes   are   viviparous.     Impregnation   may 
take  place  in  the  oviduct  or  in  the  uterus.     In  the  Placentalia 
the  ova  are  small  and  have  little  yolk,  whereas  in  the  Mono- 
tremes there  is  much  yolk,  as  among  the  birds.     The  segmen- 
tation in  the  placental  mammals  is  complete  but  not  neces- 
sarily equal.     A  solid  sphere  of  cells  is  formed  which  becomes 
differentiated  into  an  outer  enclosing  layer   (the  trophoblast, 
Fig.  234)  and  an  inner  mass  of  cells  (Fig.  234,  ent.).    It  is  the 
mass  of  cells  that  gives  rise  to  the  embryonic  layers,  from 
which  are  produced  the  adult  organs.     The  trophoblast^Tias 
little  or  no  part  in  the  formation  of  the  embryo  proper,  but 
has  a  part  in  forming  the  fcetal  membranes  so  important  in 
the  group.     The  steps  of  embryonic  development,  while  sim- 
ilar in  general  to  those  described  for  the  other  vertebrata,  are 
modified  by  the  absence  of  the  yolk  and  the  retention  of  the 
developing  egg  in  the  body  of  the  parent.     The  embryonic 


ZOOLOGY. 


membranes — amnion  and  allantois — occur  as  among  birds,  but 
their  fate  is  somewhat  different.  The  allantois  typically  fuses 
with  the  outer  layer  of  the  amnion  (false  amnion  Fig.  205,  am2) 
and  the  trophoblast  (see  above),  and  this  combined  tissue,  the 
chorion,  becomes  connected  with  the  wall  of  the  uterus  by 
outgrowths  or  villi.  These  become  closely  associated  with  the 


C 


FIG.  234.  Diagram  of  Segmentation  of  ovum  in  Mammals.  A,  ovum;  B,  showing 
the  early  differentiation  into  an  outer  layer  which  produces  the  trophoblast  (see  p.  437), 
and  an  inner  mass  which  produces  the  embryo;  C,  a  later  stage,  ect.,  ectodermal  por- 
tion of  embryo;  ent.,  cells  destined  to  produce  entoderm;  in.,  inner  mass  of  the  cells 
which  form  the  embryo;  o,  outer  layer  which  forms  t,  the  trophoblast. 

Questions  on  the  figures. — How  does  this  differ  from, the  segmenta- 
tion in  the  sea-urchin?  What  is  the  fate  of  the  trophoblast?  Examine 
reference  texts  and  learn  how  the  ectoderm,  entoderm,  and  mesoderm  of 
the  real  embryo  (the  inner  mass  of  cells)  are  formed. 

tissues  of  the  mothers.  This  combination  of  maternal  and 
embryonic  tissues  is  called  the  placenta,  and  is  the  character- 
istic organ  of  the  Placentalia  or  true  mammals. 

It  is  by  means  of  these  united  tissues  that  food  and  oxygen 
pass  from  the  blood  of  the  mother  into  the  blood  of  the  em- 
bryo. In  the  marsupials  the  attachment  is  very  ( slight,  and 
for  this  reason  uterine  nutrition  becomes  insufficient  relatively 
early  and  the  young  must  be  provided  for  in  some  other  way. 
The  marsupium,  in  which  the  milk  glands  open,  presents  the 
solution  of  the  problem  of  later  development  of  the  foetus. 
So  at  birth  the  immature  young  of  marsupials  are  placed  by 
the  mother  in  the  pouch.  It  is  important  to  remember  that 


MAMMALIA. 


439 


the  blood  vessels  of  the  mother  and  the  embryo  are  not  con- 
tinuous. The  blood  of  the  embryo  is  developed  in  the  same 
manner  as  its  other  tissues  and  is  not  derived  from  the  mother 
directly. 

452.  Classification  of  Mammals. — In  the  introductory 
survey  in  §  437  the  three  subclasses  have  already  been  out- 
lined. 

Subclass  I.  Ornithodelphia  or  Monotremata. — Mammals 
whose  mammary  glands  have  no  nipples;  they  lay  eggs  with 
abundant  yolk,  which  are  hatched  outside  the  body,  as  in  birds. 
The  alimentary  canal  ends  in  a  cloaca.  One  ovary  is  some- 
times incompletely  developed  as  in  birds,  and  the  oviducts  open 
separately  into  the  vagina.  The  duck-bill  or  duck-mole  lives 
in  water,  or  burrows  in  the  banks  of  streams  or  lakes.  It  is 

FIG.  235. 


FIG.  235.     Duck-bill   (Ornithorhynchus  anatinus).     Photographed  by  Folsom. 

Questions  on  the  figure.— What  are  the  peculiarities  of  Ornithorhyn- 
chus ?    What  does  the  structure  of  its  feet  indicate  as  to  its  habits? 

eighteen  or  twenty  inches  long  and  is  covered  with  soft  fur. 
Its  eggs  are  laid  in  its  burrows.  Echidna  or  the  spiny  ant- 
eater,  lives  in  rocky  places  and  captures  ants  by  means  of  its 
slender,  sticky  tongue.  They  are  confined  to  Australasia  and 
are  interesting  chiefly  because  of  their  likeness  to  the  reptile? 
and  birds  (Fig.  235). 


44°  ZOOLOGY. 

Subclass  II.  Didelphia  or  Marsupialia. — Mammals  with 
nipples ;  these  occur  in  the  pouch  on  the  ventral  surface  of  the 
body  in  which  the  immature  young  are  placed  at  birth.  The 
young  are  too  immature  to  suck  voluntarily  at  first,  and  milk 
is  forced  into  the  mouth  by  the  action  of  muscles  about  the 
gland.  The  pouch  is  usually  supported  by  two  bones  attached 
to  the  pubis  and  running  forward.  There  are  two  oviducts, 
two  uteri,  and  even  the  vaginae  may  be  paired  (Figs.  50,  60). 
Many  different  types  are  included  in  this  group.  Some  are 
rat-like  in  appearance,  others  similar  to  the  dog,  others  to  the 
bear.  Some  are  herbivorous,  some  carnivorous,  others  insec- 
tivorous. With  the  exception  of  the  American  opossum  fam- 
ily, the  living  species  are  native  of  Australasia.  Fossil  mar- 
supials are  found  in  all  parts  of  the  world,  showing  that  they 
are  an  ancient  type  of  mammals  which  have  become  extinct 
except  in  the  places  cited.  Many  of  the  fossil  forms  were  of 
gigantic  size.  The  largest  living  species  is  the  kangaroo. 

Subclass  III.  Monodclphia  or  Placentalia. — Mammals  in 
which  the  young  are  connected  to  the  wall  of  the  maternal 
uterus  by  means  of  a  placenta  (see  §  451 )  ;  two  oviducts ;  uteri 
more  or  less  united  into  one;  vagina  single;  no  cloaca;  no 
marsupium.  The  segmentation  of  the  ovum  is  total. 

The  following  key  will  assist  the  student  to  get  a  view  of 
the  principal  orders  of  the  placental  mammals : 

Teeth  wanting,  or  without  enamel Edentata. 

Teeth  with  enamel. 

Hind  limbs  wanting. 

Front  appendages  with  elbow  joint Sirenia. 

Front  appendages  without  elbow  joint Cetacea. 

Hind  limbs  developed. 

Nails  of  the  digits  hoof-like Ungulata. 

Nails  claw-like. 

The  front  limbs  modified  to  form  wings, 

Cheiroptera. 
No  wings. 

Thumbs  not  opposable. 

Incisors  and  canines  small, 

Insectivora. 


MAMMALIA.  44! 

Incisors  chisel-shape  and  canines 

wanting    Rodentia. 

Canines  large;  other  teeth  often 

pointed    Carnivora. 

Thumbs   opposable    Primates. 

Order  i.  Edentata. — Placentalia  in  which  the  teeth  are 
absent  or  imperfect,  being  destitute  of  enamel  and  true  roots. 
They  are  found  both  in  the  Old  World  and  in  the  New,  espe- 
cially in  the  tropics  of  the  southern  hemisphere.  The  chief 
representatives  are  the  sloths,  the  hairy  ant-eater  and  the  arma- 
dillo of  South  America,  and  the  scaly  ant-eaters  of  Asia  and 
Africa.  The  sloths  are  sluggish  vegetarians  living  in  the 
trees,  on  the  branches  of  which  they  hang  or  climb,  back  down- 
ward, by  means  of  their  long  curved  claws.  The  ant-eaters 
are  almost  wholly  devoid  of  teeth,  but  have  narrow  extensible 
tongues  which  they  project  into  ant-holes,  capturing  the  ants 
by  the  sticky  saliva.  The  group  is  primitive  and  degenerate, 
and  furnishes  a  noteworthy  exception  to  the  statement  that  the 
mammals  lack  an  external  skeleton.  Overlapping  bony  scales, 
or  plates  in  the  form  of  rings,  may  furnish  a  complete  armor 
by  means  of  which  they  are  kept  from  extermination  in  spite 
of  their  inoffensive,  sluggish  habits. 

Order  2.  Sirenia. — A  small  group  of  aquatic  Placentalia, 
more  or  less  whale-like  in  form.  They  are  sluggish,  ungainly, 
vegetable  feeders.  They  have  no  posterior  appendages  and 
the  anterior  are  flipper-like,  though  capable  of  bending  at  the 
elbow.  They  live  near  the  shore  and  are  represented  by  two 
living  genera,  the  sea-cow  of  our  own  eastern  shores  (Mana- 
tee}, and  the  Dugong  of  the  Indian  Ocean. 

Order  j.  Cetacea  (Whales,  Porpoises,  etc.}. — The  Cetacea 
are  aquatic  mammals  with  a  fish-like  body.  There  are  no 
posterior  appendages,  and  the  anterior  act  as  paddles,  being 
without  joint.  The  tail  is  horizontally  expanded  into  a  pow- 
erful paddle,  and  a  dorsal  fin  is  usually  present.  Teeth  are 
present  in  the  embryo,  but  may  be  lost  or  replaced  by  "  whale- 
bone "  in  the  adult.  The  stomach  is  chambered.  The  two 


442  ZOOLOGY. 

mammae  are  posterior.  Hair  is  very  scant,  but  the  layer  of 
fat  or  "  blubber  "  beneath  the  skin  is  very  thick,  and  serves  as 
a  non-conductor  of  heat. 

The  whales  are  the  largest  living  animals.  The  largest  of 
these,  the  Greenland  whale,  may  attain  a  length  of  seventy-five 
feet  or  more.  It  must  be  remembered  that  the  whales  are  air- 
breathers,  and  therefore  must  come  to  the  surface  to  breathe 
or  "  blow."  The  Cetacea  prey  on  the  smaller  swimming  or 
floating  animals  found  in  the  ocean,  as  fish,  squid,  Crustacea, 
etc.  Whales  are  principally  sought  for  their  fat  and  baleen, 
or  whale-bone. 

Order  4.  Ungnlata  (hoofed  animals). — This  order  in- 
cludes a  great  number  of  animals,  chiefly  herbivorous,  that 
walk  on  their  toes.  In  these  forms  the  horny  growth  which 
we  have  so  frequently  found  in  vertebrates  at  the  end  of  the 
digits  takes  the  form  of  a  hoof.  Toes  are  usually  not  more 
than  four  in  number.  The  canine  teeth  are  small  or  absent. 
The  following  suborders  are  important. 

Suborder  (a)  Artiodactyla  (even-toed). — These  are  ungu- 
lates with  toes  reduced  to  four  or  two.  The  third  and  fourth 
toes  persist  and  bear  the  weight  of  the  animal,  and  the  second 
and  fifth,  if  present,  may  or  may  not  touch  the  ground.  The 
mammae  are  distributed  along  the  entire  abdomen  or  are  con- 
fined to  the  pelvic  region.  The  ruminants,  as  the  ox,  the 
camel,  sheep  and  deer,  and  the  non-ruminants,  as  the  swine 
and  the  hippopotamus,  are  representatives  of  the  group.  Here 
belong  some  of  man's  most  useful  food-animals. 

Suborder  (b)  Perissodactyla  (odd-toed). — These  are 
characterized  by  the  fact  that  the  weight  of  the  body  rests  on 
the  third  or  middle  toe,  the  others  being  more  or  less  reduced. 
The  stomach  is  simple.  No  proboscis.  The  mammae  are  few 
and  confined  to  the  pelvic  region.  The  most  common  exam- 
ples are  the  horse  and  its  allies,  in  which  the  third  is  the  only 
digit,  and  the  rhinoceros,  which  has  the  second  and  fourth,  as 
well.  It  is  known  that  the  remote  ancestors  of  the  horse  had 
a  second  and  a  fourth  toe  where  only  splints  occur  now,  and 


MAMMALIA. 

even  a  first  and  a  fifth,  where  now  there  is  no  trace  of  either. 
Here  belong  also  the  ass,  the  zebra  and  the  tapir. 

Suborder  (c)  Proboscidea  (with  proboscis). — Two  living 
and  many  extinct  species  of  huge  Placentalia  with  five  digits, 
each  with  a  distinct  hoof.  The  nose  is  much  developed  into 
a  prehensile  organ,  with  corresponding  changes  in  the  skull 
for  attachment  of  muscles.  The  upper  incisors  grow  enor- 
mously, forming  the  tusks  characteristic  of  the  group.  No 
canines;  molars  very  complex.  Two  thoracic  teats.  The 
largest  of  the  land  mammals,  the  elephants  and  the  extinct 
mastodon  and  mammoth,  belong  here.  They  are  now  con- 
fined to  the  tropical  regions  of  Asia  and  Africa,  though  in 
geological  times  they  seem  to  have  had  a  world- wide  range. 
The  tusks  of  species  of  elephants,  both  living  and  extinct,  fur- 
nish the. ivory  of  commerce. 

Order  5.  Carnivore,  (flesh-eaters). — The  Carnivora  are 
four-  or  five-toed  animals  with  the  digits  ending  in  claws.  The 
canines  are  well  developed,  strong  and  curved.  The  other 
teeth  are  often  pointed  and  adapted  to  holding  or  tearing. 
Muscles  of  mastication  are  especially  well  developed.  Mammae 
are  numerous,  occurring  along  the  entire  abdomen.  There  are 
two  types  of  Carnivora — terrestrial  and  marine.  To  the  first 
belong  the  bear  family,  which  is  perhaps  the  least  specialized 
group ;  the  dog  family,  including  dogs,  wolves,  foxes,  jackals ; 
the  cat  family,  including  lions,  tigers,  leopards ;  many  fur- 
bearing  animals — as  otters,  weasels,  minks,  martens,  etc. 
The  seals  and  walruses  belong  to  the  marine  group.  In  these 
forms  the  appendages  have  become  adapted  to  the  water  habit, 
the  digits  bearing  intervening  webs.  The  order  embraces 
many  very  powerful  and  intelligent  animals  which  are  well 
adapted  to  win  in  the  struggle  for  life,  if  it  were  not  for  human 
interference.  In  the  presence  of  man,  however,  all  those  which 
are  not  suited  to  domestication  are  gradually  disappearing; 
some  because  of  their  dangerous  qualities,  others  because  of 
the  value  of  their  products.  The  group  is  not  used  to  any 
considerable  extent  as  food. 


444  ZOOLOGY. 

Order  6.  Rodentia  {gnawing  animals). — The  rodents  are 
small  mammals  with  clawed  digits.  They  have  no  canine  teeth, 
but  have  well-developed  chisel-shaped  incisors  which  continue 
to  grow  as  they  are  worn  at  the  extremity.  The  chisel  edge  is 
preserved  by  the  fact  that  the  enamel  is  chiefly  in  front,  and 
the  exposed,  softer  dentine  behind  is  WOKH  away  more  rapidly 
by  being  used.  The  brain  is  smooth.  The  mammae  are 
abdominal.  The  rodents  have  world-wide  distribution,  and  are 
especially  well  represented  in  North  America.  The  principal 
types  are  the  rats  and  mice,  many  of  which  are  close  followers 
of  civilized  man ;  squirrels  and  prairie  dogs,  beavers,  hares  and 
rabbits,  and  porcupines.  The  rodents  feed  on  vegetable  diet, 
and  are  destructive  of  many  plants  and  grains  which  man  uses 
for  food.  Notwithstanding  man's  efforts  to  destroy  them  their 
remarkable  power  of  reproduction  enables  the  more  aggressive 

FIG.  236. 


FIG.  236.     The  Jumping  Rat  (Perodipus  richardsoni),  adult  male.     Photographed  from 
life  by  Dr.  R.  W.  Shufeldt. 

Questions  on  the  figure. — What  order  of  mammals  is  illustrated  by 
this  form?  What  explanations  are  offered  as  to  the  cause  of  the  light 
color  of  the  belly  and  the  dark  color  of  the  backs  of  animals?  Of  what 
conceivable  advantage  is  the  difference  in  coloration?  How  does  the  tail 
of  this  species  compare  with  that  of  our  common  rat? 


MAMMALIA. 


445 


families  to  hold  their  own.  Serious  charges  are  brought 
against  the  rats  of  being  the  carriers  of  various  diseases,  e.  g., 
of  the  bubonic  plague.  (Figs.  236-238.) 

FIG.  237. 


FIG.   237.     The    Fox    Squirrel    (Sciurus    ludoricianus).     Photographed    by    Folsom. 

FIG.  238. 


FIG.  238.     Porcupine    (Erethizon}.     By    Dr.    J.    W.    Folsom. 

Questions    on    the    figure. — To    what    order    of    mammals    does    the 
porcupine  belong?     What  are  its  peculiarities  of  habit  and  structure? 

Order    7.  Insectivora    (insect-feeders). — These    are    small 
mammals  with  clawed  digits,  which  feed  on  insects  and  other 


446 


ZOOLOGY. 


small  invertebrates.  The  brain  is  small  and  smooth.  The  in- 
cisors are  small.  Many  burrow,  and  have  special  adaptations 
for  such  a  life;  among  these  one  of  the  most  interesting  is 
the  degeneration  of  the  eyes.  The  moles,  shrews  and  hedge- 
hogs are  the  chief  representatives. 

FIG.  239. 


FIG.  239.     Flying  Fox   (Pteropus).     U.  S.   Dept.  Agriculture  Year-book,    1898. 

Questions  on  the  figure. — What  is  the  structure  and  arrangement  of 
the  wings  in  such  a  form  as  this?  To  what  order  of  mammals  does  this 
type  belong? 

Order  8.  Cheiroptera  (hand-winged;  the  bats). — Mam- 
mals in  which  flight  is  made  possible  by  a  web  or  fold  of  the 
skin  stretching  between  the  much  extended  fingers  of  the  anter- 
ior appendages ;  between  the  arm,  body,  and  the  hind  legs ;  and 
thence  even  to  the  tail.  The  thumb  and  posterior  digits  are 
clawed.  The  sternum  has  a  keel  as  in  birds.  The  mammae  are 
thoracic.  The  bats  are  the  only  mammals  capable  of  active 
flight.  They  feed  on  insects  or  fruits.  One  species  is  known 
to  suck  blood.  (Fig.  239.) 


MAMMALIA. 


447 


Order  p.  Primates  (first  or  highest}. — With  the  exception 
of  man  the  primates  are  arboreal  in  habit.  In  adaptation  to 
this  the  thumb  and  great  toe  are  usually  opposable  to  the 
other  digits,  as  in  the  human  hand.  The  digits  are  armed 
with  nails  which  are  in  some  cases  claw-like.  The  cerebrum  is 
large  and  in  higher  forms  much  convoluted.  Mammae  chiefly 
thoracic  (abdominal  in  some  lower  forms).  The  group  em- 

FIG.  240. 


FIG.  240.     Hand   and   foot   of   Chimpanzee.     From.  Home   and    Country   Magazine. 

Questions  on  the  figure. — Which  is  hand,  and  which  foot?  In  what  re- 
spects do  they  differ?  How  do  they  differ  from  the  hand  and  foot  of 
man?  In  which  is  the  difference  from  the  human  condition  greater? 
What  is  the  functional  meaning  of  these  differences? 

braces  the  lemurs,  monkeys  of  various  kinds,  baboons  with 
non-prehensile  tails,  the  tailless  apes  most  like  man,  and  man 
himself.  Man  is  to  be  distinguished  from  the  higher  apes  by 
having  shorter  arms,  better  developed  legs,  more  erect  pos- 
ture, non-opposable  great  toe;  in  the  greater  size  and  com- 
plexity of  the  brain,  and  especially  in  mental  and  moral  capa- 
bilities (Fig.  240). 

The  primates  below  man  are  found  chiefly  in  tropical  regions 
and  more  abundantly  south  of  the  equator.  They  feed  largely 
on  fruit  and  insects,  though  some  eat  birds  and  other  small 
animals.  Many  of  them  are  social,  or  at  least  gregarious,  and 


448  ZOOLOGY. 

their  habits  of  life  are  interesting  and  suggestive  in  a  high 
degree,  when  we  consider  their  possible  relation  to  the  human 
species.  In  physical  structure  man  differs  less  from  the  gorilla 
and  the  chimpanzee  than  these  from  the  monkeys  of  South 
America.  Man  is  the  only  primate  native  to  this  country,  and 
doubtless  man  came  to  America  from  Asia. 

453.  Additional  Notes  on  the  Habits  of  Mammals. — We 
have  seen  that  mammals  have  succeeded  in  occupying  in  over- 
whelming numbers  the  land,  in  much  less  degree  the  water,  and 
least  of  all  the  air.  We  have  classified  them  as  insectivorous 
(moles,  ant-eaters,  and  bats)  ;  carnivorous,  as  the  beasts  of 
prey;  herbivorous,  as  the  hoofed  animals,  rodents,  and  kan- 
garoo; or  omnivorous,  as  the  pigs  and  man.  They  are  very 
versatile  and  have  dominated  the  earth  since  the  tertiary 
epoch  when  they  supplanted  the  immense  reptiles  of  the  earlier 
ages.  One  of  the  most  noteworthy  facts  in  connection  with 
the  group  is  the  degree  of  care  given  to  the  young  by  the  par- 
ents, especially  the  mother.  This  is  true  not  merely  in  gesta- 
tion but  after  birth  in  the  attendance  of  the  mother  to  the 
needs  of  the  young,  both  in  supplying  food  and  in  protecting 
from  danger.  It  must  be  remembered  that  this  is  done  at  the 
expense  of  the  parent's  safety.  It  means  that  the  species  may 
be  kept  alive  by  the  birth  of  a  smaller  number  of  young,  be- 
cause more  will  reach  maturity  than  if  left  early  to  shift  for 
themselves;  and,  further,  that  a  higher  development  of  the 
young  becomes  possible  owing  to  the  increased  length  of  youth. 
The  degree  of  development  at  birth  is  quite  variable.  In  a 
general  way  it  is  less  in  the  case  of  those  whose  parents  can 
best  protect  the  helpless  young.  For  example  the  young  of  the 
Carnivora  and  of  the  Primates  are  much  less  able  to  take  care 
of  themselves  at  birth  than  the  young  of  the  Herbivora.  Many 
biologists  have  called  attention  to  the  fact  that  the  greater  care 
of  the  young  implies  higher  instincts  and  intelligence  on  the 
part  of  the  parent.  This  is  subject  to  the  action  of  natural 
selection  as  an  advantageous  characteristic.  In  turn  a  longer 
youth  or  period  of  development  is  demanded  for  the  maturing 


MAMMALIA.  449 

of  these  higher  instincts,  thus  making  a  new  demand  on  the 
parent  for  care  and  training. 

The  social  instinct  is  well  represented  among  mammals. 
This  may  vary  from  collection  in  mere  shoals  or  herds  where 
food  is  abundant,  to  groups  organized  for  offense  and  defense 
and  for  work, — as  wolves,  cleer,  beavers.  Indiscriminate 
mating  is  the  rule,  yet  in  some  instances  strict  monogamy  is 
found.  In  many  cases  mates  are  won  by  force,  and  this  tends 
to  result  in  the  selection  and  propagation  of  the  strong.  The 
struggle  among  the  males  is  accompanied  by  the  development 
in  them  of  numerous  structures  which  the  females  do  not  pos- 
sess at  all  or  at  least  in  such  degree : — as  antlers,  horns,  tusks, 
manes, — and  greater  size. 

It  is  in  the  higher  mammals  that  one  finds  the  greatest  dis- 
play of  intelligence  to  be  seen  in  the  animal  kingdom,  and  it 
is  in  man  that  intelligence  and  reason — whose  beginnings  in 
animals  no  one  can  mark — find  their  culmination.  That  these 
high  qualities  are  closely  correlated  with  the  great  development 
of  the  brain  there  can  be  no  doubt.  The  great  progress  of  man 
in  getting  mastery  of  the  earth  is  one  of  the  most  interesting 
aspects  of  the  same  general  problem  of  evolution  and  adapta- 
tion which  gives  unity  to  the  subject  matter  of  zoology. 
Thus  the  sciences  which  pertain  to  man  in  all  his  various  in- 
terests have'  in  some  measure  their  foundation  in  the  science 
of  zoology. 

454.  Supplementary  Topics  for  Field  and  Library. 

1.  Enumerate  the  native  species  of  mammals  known  by  you 
to  be  found  in  your  locality,  and  determine  to  which  of  the 
orders  of  mammals  they  belong.     Are  you  impressed  that  the 
number  of  native  species  of  mammals  is  large  or  small  as  com- 
pared with  other  animals  ?  Are  the  individuals  of  these  species 
numerous  or  not  ?  How  do  you  account  for  the  facts  you  have 
discovered  ? 

2.  Enumerate  the  species  of  domestic  mammals  in  your  lo- 
cality.    Are  they  related  to  any  of  the  native  species?   Trace 

30 


45°  ZOOLOGY. 

the  history  of  some  of  the  most  important  domestic  types. 
What  do  you  know  of  the  mammals  domesticated  in  other 
parts  of  the  earth? 

3.  Make  a  report  on  the  ruminants :  their  habits,  their  dis- 
tribution over  the  earth,  and  their  uses  to  man. 

4.  What  is  known  of  the  geological  history  of  the  horse 
family?   Is  the  horse  a  native  of  America? 

5.  What  is  the  history  of  the  introduction  of  rabbits  into 
Australia  ?     Can  you  cite  any  similar  history  of  rodents  in  this 
country  ? 

6.  Report  on  furs  and  fur-bearing  animals.     What  is  the 
present  state  of  the  seal  fisheries  of  our  Pacific  coast?   What 
steps  are  necessary  to  the  preservation  of  the  seal?   On  what 
zoological  grounds  are  these  steps  necessary? 

7.  Make  a  report  on  the  habits  and  instincts  of  the  beaver. 
Describe  the  nature  of  its  social  life. 

8.  Report  on  the  condition  of  primitive  man.     Which  of 
man's  instincts  have  been  of  most  use  to  him  in  his  develop- 
ment?  What  are  the  principal  faculties  separating  him  from 
the  other  primates  ?  Do  all  men  possess  these  in  equal  degree  ? 
What  is  the  distribution  of  the  principal  races  or  varieties  of 
the  human  species?    What  are  the  chief  differences  between 
these  races? 


F    THE 

UNIVERSITY 

OF 


CHAPTER    XXV. 

EXERCISES  IN  COMPARATIVE  PHYSIOLOGY,  MORPHOLOGY, 
AND  ECOLOGY. 

455.  Now  that  the  student  has  studied  in  some  detail  the 
work  which  even  the  simplest  organisms  must  perform,  the 
organs  by  means  of  which  this  necessary  work  is  done  in  some 
of  the  principal  types,  and  the  relations  which  animals  assume 
to  each  other  and  to  the  environment  in  general,  it  is  desirable 
that  he  should  bring  these  facts  into  such  relations  that  they 
may  be  compared.  The  likenesses,  the  unlikenesses,  and  the 
progressive  differentiation  are  thus  brought  into  clear  relief. 
The  following  outline  exercises  are  intended  to  guide  the  stu- 
dent in  this  task.  They  are  by  no  means  exhaustive,  but  will 
suggest  the  principal  points  most  essential  to  such  a  resume. 
The  laboratory  notes,  the  text-book,  and  all  the  reference  books 
at  his  command,  should  be  used  by  the  student.  The  teacher 
should  require  the  student  to  be  able  to  cite  his  authority  for 
all  important  statements  not  his  own  and,  if  possible,  require 
corroboration  by  reference  to  more  than  one  authority.  The 
author  has  found  that  tables  with  parallel  columns  such  as 
those  on  pages  340  and  343  furnish  an  economical  and  other- 
wise satisfactory  mode  of  displaying  the  results  of  these  studies. 

I.  Fundamental   Form    (Promorphology) .  —  Indicate    for 
each  of  the  important  phyla,  or  for  chosen  representatives  of 
them,  the  following  matters  of  general  form:   kind  of  sym- 
metry represented  and  the  perfection  of  its  development;  the 
degree  and  character  of  segmentation;  the  position,  number, 
character,  and  arrangement  of  the  appendages;  the  external 
and  the  internal  evidences  of  cephalization ;  the  relation  of  the 
principal  organs  of  the  animal  to  the  horizontal  and  vertical 
planes. 

II.  Physiology  and  Morphology. — Compare  the  mode  of 

45 i 


45 2  ZOOLOGY. 

performing  the  following  functions  and  the  organs  used 
therein,  in  all  the  principal  animal  phyla. 

i.  The  capture  of  food:  the  method;  the  organs  devoted  to 
it ;  and  the  relation  of  these  to  the  nature  of  the  food  used. 

2..  Digestion;  physical  and  chemical. 

3.  Circulation,  as  pertaining  both  to  the  nature  of  the  circu- 
lating fluid  and  to  the  organs  moving  it;  the  relation  of  the 
whole  process  to  the  organs  and   function  of  digestion  and 
respiration  in  the  types  chosen. 

4.  Respiration :  the  medium  containing  the  oxygen,  and  the 
contrivances  for  securing  it. 

5.  Excretion :  note  and  classify  the  chief  modes  of  elimin- 
ating waste  materials  observed  in  the  animal  phyla. 

6.  The  body  cavity   (coelom)   in  relation  to  digestion,  cir- 
culation and  excretion. 

7.  Physical   support   and  protection    (skeletal   structures)  ; 
their  position,  structure,  and  mode  of  formation. 

8.  Motion  and  locomotion :   degree  of  each ;  relation  of  the 
muscular  or  contractile  elements  to  the  skeletal.     The  medium 
used  in  locomotion ;  the  principal  special  devices  in  each  group 
for  the  solution  of  the  problems  presented  by  the  medium. 

9.  Sensitiveness :  the  kinds  of  stimuli  to  which  the  organ- 
isms in  the  various  groups  react;  the  differences  in  the  differ- 
ent phyla  in  each  of  the  various  classes  of  sense  organs,  as  to 
structure,  position,  and  manner  of  action ;  the  number,  position 
and  perfection  of  the  nerve  centres ;  and  the  relation  of  the 
nerve  centres  to  the  sense  organs  and  to  the  muscles. 

10.  Reproduction.     The  various  methods,  and  the  special 
ends  accomplished  by  each ;  rate ;  number  of  offspring ;  paren- 
tal care;  sex  dimorphism;  alternation  of  generation;  partheno- 
genesis. 

III.  Ecology  and  Adaptations  to  the  Environment. — Com- 
pare the  animal  groups  from  the  following  points  of  view. 

1 .  General  habitat :  aquatic,  fresh  or  salt  water ;  terrestrial ; 
aerial. 

2.  Migration  or  other  special  means  of  effecting  distribution 
from  the  point  of  origin. 


EXERCISES.  453 

3.  Degree  of  connection,  organic  or  social,  between  the  in- 
dividuals of  a  species;  gregarious,  social  and  communal  life; 
resulting  social  qualities;  degree  of  division  of  labor;  poly- 
morphism. 

4.  Power  of  regenerating  lost  parts. 

5.  Growth;    rate,    and    ultimate    size;    longevity.     Special 
hindrances  to  growth. 

6.  Relation  to  human  welfare :  use  as  food ;  effects  on  crops 
and  domestic  animals ;  the  production  or  dissemination  of  dis- 
ease in  man;  capability  of  domestication;  other  qualities  help- 
ful or  hurtful  to  human  interests.     Which  phyla  furnish  spe- 
cies susceptible  of  domestication? 

7.  Diseases  among  animals  other  than  man. 

8.  Coloration  :  pigments,  internal  and  external ;  other  modes 
of  producing  color;  location  of  the  color;  supposed  uses. 

9.  Principal  methods  of  avoiding  or  surviving  unfavorable 
periods,  as  cold,  drouth,  and  the  like. 

10.  Qualities  of  offense  and  defense. 

11.  Protective   resemblance   and   mimicry.     Other   passive 
modes  of  protection. 

12.  Parasitism  and  the  degree  of. degeneracy  resulting  from 
il. 

IV.  Geographical  Distribution. — Select  several  representa- 
tive species  from  each  animal  phylum  and  learn  everything 
you  can  concerning  their  distribution  on  the  earth.  Are  they 
local  species  or  cosmopolitan  species?  What  seems  to  be  the 
reason  for  the  fact?  Are  all  the  phyla  cosmopolitan?  Com- 
pare the  phyla  from  the  following  points  of  view : 

1.  The    facilities    for    migration.     The    special    modes    of 
migration,  both  active  and  passive. 

2.  What  are  the  principal  barriers  to  migration  and  distri- 
bution in  the  case  of  the  representatives  chosen  for  study? 

3.  Find  instances  of  species  of  animals  apparently  closely 
related,  with  different  geographical  distribution.     Compare, 
for  example,  the  species  of  hares  and  rabbits  found  in  North 
America ;  the  species  of  lynx ;  of  bears ;  of  the  alligators ;  spe- 
cies of  Unio;  of  the  lobster;  of  the  genus  Equus. 


454  ZOOLOGY. 

4.  Make  a  local  map  of  the  region  about  your  school  on  a 
large  scale.  Show  all  ponds,  streams,  lakes,  marshes,  mead- 
ows, uplands,  forests,  etc.  Show  by  suitable  symbols  where 
various  species  of  animals  are  to  be  found  with  reasonable 
certainty.  Keep  in  a  note-book  belonging  to  the  laboratory 
a  memorandum  of  each  new  species  found,  and  of  a  new  local- 
ity for  a  known  species.  In  time  the  map  and  the  note-book 
will  be  a  good  account  of  the  local  distribution  of  species. 


APPENDIX. 
SUGGESTIONS  TO  TEACHERS. 

1.  The  Relation  of  the  Descriptive  Work  to  that  of  the 
Laboratory  and  Field. — If  time  were  of  no  consideration,  it 
would  perhaps  be  desirable  for  each  student  to  get  all  his  infor- 
mation concerning  animals  at  first   hand.     Even  under  this 
most  favorable  assumption,  however,  his  information  would 
have  a  detached  and  unrelated  quality  which  can  only  be  cor- 
rected by  lectures  or  text-book.     This  indicates  the  author's 
view  of  the  purpose  of  the  body  of  the  text.     It  is  to  conserve 
the  pupil's  time  and  to  unify  his  own  necessarily  scattered 
observations  in  such  a  way  as  to  give  them  a  vital  and  perma- 
nent interest.     For  this  end  the  practical  work  in  each  phylum 
of  animals  should  precede  the  descriptive  and  not  be  used 
merely  to  illustrate  it.     The  text-book  instruction  and  library 
references  should  have  a  much  wider  scope  and  fuller  illustra- 
tive detail  than  is  possible  in  the  laboratory. 

2.  The  Nature  of  the  Practical  Work. — Personally  the 
author  has  little  sympathy  with  the  sentiment,   so  much  in 
evidence  of  recent  years,  that  the  most  bizarre  and  superficially 
interesting  phenomena  are  the  ones  most  likely  to  lead  to  good 
educational  results.     These  may  be  well  enough  in  their  place, 
but   their   best   possible   place   when   not   abused   is   only   to 
heighten  interest  in  the  more  important  relations  and  phenom- 
ena of  animal  life.     The  animal  furnishes  interesting  and  im- 
portant facts  in  two  essential  relations :    ( i )  the  internal,  in 
connection   with  which   we   are   concerned   equally  with   the 
fundamental  structure  and  with  its  relation  to  the  work  to  be 
done  by  the  organism;  and  (2)  the  external,  in  which  we  are 
interested  in  this  same  work  done  by  the  parts  of  the  organ- 
ism, but  in  relation  to  the  conditions  on  the  outside  of  the 
animal.     Physiology  is  thus  the  connecting  link  between  mor- 

455 


456  ZOOLOGY. 

phology  and  ecology.  The  exercises  of  this  book  have  been 
arranged  in  the  main  to  lead  the  student  to  see  first  what  the 
animal  types  do;  secondly,  the  relation  of  this  activity  to  the 
outside  world;  and  thirdly,  the  more  important  structures  by 
which  this  relation  is  maintained.  The  practical  work  should 
then  be  (i)  physiological,  which  involves  both  the  field  and 
the  laboratory;  (2)  ecological,  chiefly  in  the  field;  and  (3) 
morphological,  chiefly  in  the  laboratory.  In  each  case  the 
student  should  be  caused  to  take  the  attitude  of  answering 
questions,  preferably  of  his  own  asking,  rather  than  of  verify- 
ing descriptions.  The  laboratory  outlines  seek  to  raise  ques- 
tions rather  than  to  supply  answers. 

3.  The  Order  of  Work  and  the  Time  to  be  Given  (see 
table). — The  author  has  arranged  the  matter  in  the  book  as  it 
appears  to  him  it  should  be  presented  if  the  various  organisms 
were  always  available  when  needed,  a  condition  which  every 
teacher  knows  to  be  contrary  to  fact.  Everything  considered, 
the  author  thinks  the  best  results  may  be  had  by  beginning  the 
year's  work  in  the  spring  term  and  finishing  it  in  the  autumn 
term  of  the  next  year.  No  arrangement  of  courses  can  be 
best  for  all,  but  the  following  tables  may  be  suggestive  as  to 
the  order  of  treatment,  time  to  be  devoted  to  various  types,  and 
the  like.  "  Practical  "  is  meant  to  include  field  work,  labora- 
tory work,  demonstrations,  and  themes  worked  out  in  the 
library.  A  whole  year's  work  is  supposed  to  embrace  not  less 
than  five  exercises  per  week  for  about  thirty-six  weeks.  The 
author  has  purposely  placed  at  the  disposal  of  the  teacher  in 
this  text-book  about  three  times  as  much  work  as  can  be  done 
well  in  the  allotted  time.  The  purpose  of  this  is  that  each 
teacher  may  have  the  privilege  of  electing  material  most  suited 
to  his  special  circumstances,  and  yet  have  before  him  an  ideal 
of  what  a  thorough  elementary  course  should  cover. 

For  a  course  covering  one-half  year  and  given  in  the  spring 
term  the  order  would  be  about  that  of  I  and  the  time  about 
as  in  III.  In  a  course  of  one-half  year  the  bulk  of  the  matter 
in  fine  print  should  be  omitted,  or  used  in  just  such  measure 


SUGGESTIONS   TO   TEACHERS 


457 


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45  <S  ZOOLOGY. 

as  time  will  permit.  The  marine  forms,  which  the  majority 
of  schools  will  have  to  study  from  preserved  materials,  and 
the  general  part  of  the  text  (chapters  I  to  VIII)  should  be 
studied  in  the  winter  months  when  the  local  animals  are  least 
active.  In  connection  with  the  review  in  chapter  XXV,  chap- 
ters I  to  VIII  should  be  reread  by  the  student.  Such  a  review 
will  be  especially  helpful  after  the  student  has  a  larger  body  of 
zoological  details  at  his  command. 

4.  The  Laboratory  and  its  Equipment. —  (a)  The  labora- 
tory or  work  room  should  be  well  lighted,  and  supplied  with 
flat-topped  tables ;  the  plainer,  the  better.  These  should  be  29 
to  30  inches  in  height.  If  possible  each  student  should  have 
a  drawer  where  he  may  keep  his  instruments  and  records. 
Sinks  with  running  water  are  of  course  very  desirable.  Slop- 
jars  of  earthenware  should  be  provided  for  refuse  dissections. 
and  the  like. 

(b)  There  should  also  be  another  room  in  which  living 
animals  may  be  kept.  Very  often  a  part  of  the  basement  with 
south  exposure  may  be  utilized  for  this  purpose.  The  tem- 
perature should  not  fall  to  the  freezing  point,  nor  rise  unduly 
when  the  furnace  is  heated.  In  such  a  room  as  this  many  ani- 
mals may  be  kept  much  beyond  the  period  when  they  disappear 
outside.  Fruit  jars,  tumblers,  shallow  glass  or  crockery 
dishes,  and,  best  of  all,  battery-jars  of  various  sizes  should  be 
accumulated  here.  With  a  little  ingenuity  aquarium  vessels 
of  good  size,  with  glass  sides,  may  be  made  by  means  of  good 
quality  of  pine  boxes,  putty,  and  panes  of  glass.  A  square 
may  be  taken  from  the  middle  of  two  opposite  sides  of  such  a 
box  and  the  window  inserted  in  such  a  way  as  to  give  good 
illumination  of  the  interior.  Running  water  is  even  more  of 
a  necessity  here  than  in  the  laboratory.  A  few  bell  jars,  wire 
gauze  cages  for  insects,  boxes  of  various  kinds  for  other  ani- 
mals complete  the  list  of  the  most  essential  features  of  a  good 
working  vivarium.  It  is  always  desirable  to  have  some  green 
water-plants  in  the  vessels  of  water  containing  aquatic  animals, 
e.  g.,  bladder-wort,  watercress,  duck  weed,  and  spirogyra. 


SUGGESTIONS    TO    TEACHERS.  459 

(c)  Instruments. — Every  teacher  should  insist  on  having  at 
least  one  good  compound   microscope.      If   microscopes  are 
supplied  to  the  class,  two  students  may  use  one  instrument 
when  a  full  supply  cannot  be  secured,  though  this  is  never  so 
satisfactory  as  to  have  one  student  to  an  instrument.     At  the 
outset  the  teacher  should  give  careful  instructions  to  the  stu- 
dent in  the  use  and  care  of  the  compound  microscope.     The 
laboratory  should  also  supply  dissecting  pans  of  heavy  tin,  six 
by  twelve  inches,  with  flaring  sides,  and  one  and  one-half 
inches  deep.     Pour  into  these  a  small  amount  of  melted  paraf- 
fin mixed  with  lampblack.     This  forms  an  excellent  bottom 
for  pinning  specimens  to  be  dissected.     There  should  also  be 
a  bone  cutter,  a  syringe  with  rubber  tubing  and  glass  canulas 
for  injecting  the  blood  vessels,  a  supply  of  small  pipettes,  a 
few  pipettes  with  large  bulb  and  a  dozen  or  so  flat-bottomed 
watch  glasses. 

Each  pupil  should  have,  in  addition,  a  good  hand  lens:  a 
scalpel;  a  pair  of  fine-pointed  scissors;  a  pair  of  forceps;  a 
probe;  dissecting  needles;  a  small  supply  of  glass  slides  and 
cover-glasses. 

(d)  Reagents. — The  number  of  necessary  reagents  for  a 
beginners'  course  is  not  large.     The  following  are  the  most 
essential. 

Preserving  Reagents.  Alcohol. — This  is  the  most  used  of 
all  reagents.  It  is  a  preserving  fluid.  It  hardens  organic 
matter  by  withdrawing  the  water  from  it.  Commercial  alcohol 
is  usually  of  a  strength  of  about  90  to  95  per  cent.  Specimens 
should  first  be  placed  in  50  per  cent,  alcohol  and  then  in  a  day 
or  two  be  transferred  to  a  stronger  grade  (70  per  cent.). 
After  such  treatment  they  may  be  preserved  permanently  in 
a  strength  of  70  to  80  per  cent.  Plenty  of  the  preservative 
must  be  supplied,  and  care  must  be  taken  that  it  does  not  lose 
too  much  strength  by  evaporation.  Animals  must  be  opened, 
so  that  the  fluid  may  the  more  quickly  enter  the  cavities. 

Alcohol  may  be  secured  free  of  the  revenue  tax  by  incor- 
porated institutions,  by  application  to  the  collector  of  internal 


460  ZOOLOGY. 

revenue  of  the  district  in  which  the  school  is  located.  Applica- 
tion should  be  made  several  months  before  the  alcohol  is 
needed. 

Formaldehyde  has  been  much  used  in  recent  years  as  a  sub- 
stitute for  alcohol,  or  in  combination  with  it,  as  a  preservative. 
It  may  be  obtained  as  a  40  per  cent,  solution,  and  be  further 
reduced  by  adding  from  10  to  20  times  its  volume  of  water. 
This  gives  in  the  neighborhood  of  4  per  cent,  to  2  per  cent, 
solution  and  the  resulting  fluid  will  safely  preserve  materials 
through  the  term.  The  same  care  must  be  observed  as  with 
alcohol.  If  the  formol  affects  the  pupils  unpleasantly,  the 
specimens  may  be  washed  in  water  before  studying. 

Killing  Reagents. — Chloroform  is  usually  used  as  a  stupe- 
fying reagent.  Air-breathing  animals  exposed  to  its  fumes  are 
soon  rendered  unconscious,  and  die  in  a  relaxed  condition. 

Minute  water  animals  as  Hydra,  Dero,  and  the  like,  are 
often  advantageously  killed  by  sudden  immersion  in  hot  water 
or  hot  corrosive  sublimate  (saturated  solution). 

Staining  Reagents. — A  few  stains  are  of  advantage,  if  there 
is  any  attempt  to  study  tissues  or  the  Protozoa. 

Magenta  (aqueous  solution).  One  part  by  weight  of  the 
dry  magenta  or  fuchsin  in  100  parts  of  water.  Stains 
fresh  tissues  well,  but  is  not  a  nuclear  stain. 

Methyl  green;  one  per  cent,  aqueous  solution.  Add  one  part 
of  acetic  acid  to  100  parts  of  this.  The  resulting  fluid 
is  a  superior  nuclear  stain  for  elementary  work. 

Mounting  Reagents. — Water,  alcohol  of  different  strengths, 
glycerine,  and  normal  salt  solution  (%  Per  cent-  solution  of 
common  salt)  are  the  more  commonly  used  materials  for  tem- 
porary mounting  of  objects  to  be  examined  under  the  micro- 
scope. The  normal  salt  solution  is  especially  valuable  for 
delicate  fresh  tissues,  blood,  and  the  like.  The  teacher  must 
consult  works  on  microscopical  methods  for  information  about 
the  making  of  permanent  mounts. 

Beside  the  materials  mentioned  above  it  is  often  desirable  to 
have  other  substances, — as  sugar,  acids,  salts,  and  some  of  the 


SUGGESTIONS    TO    TEACHERS.  461 

oils,  as  xylol,  benzol,  and  the  like.      These  should  be  added 
gradually  as  their  necessity  and  uses  become  apparent. 

Injection  Masses. — For  the  study  of  the  veins  and  arteries 
and  other  tubular  structures  it  is  often  desirable  to  inject  into 
them  foreign  substances  which  prevent  their  collapse  and  ren- 
der them  easy  of  identification.  For  this  purpose  a  syringe 
and  some  rubber  tubing  and  small  canulas  are  necessary.  In- 
jection masses  to  be  satisfactory  should  be  fluid  when  injected 
and  be  able  to  "  set"  or  harden,  after  injection.  For  ordinary 
work  the  following  will  serve: 

1.  Starch  injection  mass: 

Dry  laundry  starch i   volume. 

2^2  per  cent,  aqueous  solution  chloral  hydrate.  .    i   volume. 

95  per  cent,  alcohol }4   volume. 

Coloring  mixture J4   volume. 

The  coloring  mixture  is  prepared  by  mixing  equal  parts  of 
95  per  cent,  alcohol,  glycerine,  and  dry  carmine  (vermilion, 
chrome  yellow  or  Prussian  blue).  The  solid  color  should  be 
ground  into  small  portions  of  the  fluids  in  a  mortar  so  that  no 
lumps  will  be  present  in  the  mass.  This  mixture  does  not  spoil 
with  age,  but  must  always  be  well  stirred  before  using  and 
the  injecting  must  be  rapidly  done,  as  the,  solids  settle  quickly. 

2.  Gum  or  Gelatine  Injection  Masses. — It  is  often  desirable 
to  have  a  mass  which  can  be  forced  through  the  finer  vessels, 
as  the  blood  capillaries,  so  that  the  arteries  and  veins  may 
both  be  filled  by  one  injection  into  an  artery  near  the  heart. 
The  following  solution  if  injected  warm  will  pass  the  capil- 
laries.    If  the  gelatine  solution  is  first  injected  and  then  fol- 
lowed by  a  starch  mass  of  a  different  color,  the  veins  will 
ultimately  contain  the  former  and  the  arteries  the  latter,  as 
the  starch  will  not  pass  the  capillaries,  and  thus  both  may  be 
easily  studied  because  of  the  contrast  in  color. 

Gelatine  solution    (i   part  gelatine  to  6  or  8  of 

water)    i  volume. 

Glycerine  carmine Yz  volume. 

Chloral  hydrate  (concentrated  solution) 

2  per  cent.,  by  weight,  of  the  entire  mass. 


42  ZOOLOGY. 

The  gelatine  should  be  soaked  in  cold  water  and  then 
slightly  heated  until  dissolved.  The  glycerine  carmine  may  be 
prepared  as  follows :  thoroughly  pulverize  and  mix  3  grams 
of  carmine  with  a  little  water,  with  enough  ammonia  added  to 
dissolve  the  carmine.  Add  50  grams  of  glycerine.  Mix  and 
filter.  Add  gradually  to  this  mixture  enough  acidulated  gly- 
cerine (glycerine  and  acetic  acid  in  the  ratio  of  10  to  i)  to 
give  a  slight  acid  reaction  to  the  carmine  glycerine  mass. 

5.  Materials  for  Study. — The  types  of  animals  needed  for 
this  course,  with  the  exception  of  the  marine  representatives, 
may  be  secured  in  almost  any  locality,  if  sought  at  the  proper 
time.  The  teacher  should  become  entirely  familiar  with  the 
common  animals  to  be  found  within  a  reasonable  distance 
from  his  school.  It  is  especially  necessary  to  know  the  life 
most  abundant  in  the  various  ponds,  lakes  and  streams.  A 
close  watch  should  be  kept  on  the  material  gathered  from  each 
place,  and  a  record  kept  of  the  various  localities  in  which  each 
useful  type  has  been  found  and  of  the  best  time  for  collection. 
In  time  the  laboratory  will  come  to  have  an  interesting  set  of 
facts,  valuable  not  merely  in  assisting  in  the  finding  of  needed 
material,  but  as  indicating  local  distribution  (see  also  §  455; 
IV,  4).  The  students  should  be  encouraged  to  make  excur- 
sions, both  with  and  without  the  teacher,  to  collect  material 
and  extend  the  knowledge  of  the  locality. 

If  for  any  reason  living  materials  cannot  be  secured  in  the 
locality  of  the  school,  preserved  specimens  of  marine,  fresh 
water,  and  terrestrial  species  may  be  secured  of  dealers.  The 
principal  are: 

Supply  department,  Marine  Biological  Laboratory,  Wood's 
Holl,  Mass.  (Preserved  materials.) 

Mr.  F.  W.  Wamsley,  Academy  Natural  Science,  Philadel- 
phia, Pa.  (Preserved  materials.) 

Henry  M.  Stephens,  Carlisle,  Penna.  (Preserved  and  living 
material.) 

Messrs.  H.  H.  and  C.  S.  Brimley,  Raleigh,  N.  C.  (Pre- 
served and  living:  frogs,  turtles,  alligators,  etc.,  in  the  winter.) 


SUGGESTIONS    TO    TEACHERS.  463 

Mr.  C.  J.  Maynard,  447  Crafts  street,  West  Newton,  Mass. 
(Living  material.) 

Mr.  A.  A.  Sphung,  North  Judson,  Indiana.  Frogs,  turtles, 
clams,  and  cray-fish  (living). 

Dr.  F.  D.  Lambert,  Tufts  College,  Mass.  (South  Harps- 
well,  Maine,  from  June  12  to  September  15).  (Preserved 
marine  material.) 

Mr.  George  K.  Cherrie,  Brooklyn  Institute  Museum,  Brook- 
lyn, N.  Y.  (Preserved  material.) 

Aquarium  Supply  Co.,  Delair,  N.  J.     (Living  material.) 

Mr.  W.  H.  Ficklin,  Central  High  School,  Kansas  City,  Mo. 
(Preserved  material.) 

Ward's  Natural  Science  Establishment,  Rochester,  N.  Y. 

Western  Natural  Science  Establishment,  Lawrence,  Kansas. 
(Preserved  material.) 

Supply  Department,  Hopkins  Seaside  Laboratory,  Stanford 
University,  California.  (Marine  material,  preserved.) 

Most  of  these  dealers  issue  price  lists  which  may  be  had 
on  application. 

In  addition  to  such  materials  as  indicated  above,  unless  the 
instructor  has  the  time  and  equipment  to  make  satisfactory 
permanent  mounts  of  microscopic  preparations,  he  should 
secure  a  few,  illustrative  of  cell  structures,  cell  division,  cleav- 
age of  ova;  also  sections  of  hydra,  of  the  earthworm,  and 
preparations  of  some  of  the  more  important  tissues  of  higher 
animals,  as  bone,  nerve  cells  and  fibres,  epithelial  tissue,  glandu- 
lar tissue  and  the  like.  Some  of  these  may  be  purchased  of  the 
dealers  in  microscopical  supplies.  They  may  usually  be  secured 
at  reasonable  rates  by  writing  to  the  biological  departments  of 
the  large  universities.  There  are  usually  advanced  students  in 
these  laboratories  who  are  glad  to  make  a  few  dollars  in  con- 
nection with  their  work.  Such  preparations  lend  a  great  deal 
of  interest  as  demonstrations  in  connection  with  the  laboratory 
work. 

The  writer's  laboratory  (Millikin  University,  Decatur,  111.) 


464  ZOOLOGY. 

will  be  able  to  furnish  to  teachers  a  limited  number  of  sets  of 
the  microscopic  slides  called  for  in  this  book. 

6.  Laboratory  Records. — For  making  these  the  student 
should  have  a  note-book  of  unruled  drawing  paper  of  good 
quality,  which  may  be  had  in  a  tablet  or  kept  as  separate  sheets 
in  an  appropriate  envelope;  and  good  drawing  pencils,  kept 
sharp,  and  of  hardness  suited  to  the  paper.  In  the  note-book 
the  student  should  keep,  concisely  and  in  an  orderly  way,  the 
record  of  all  his  observations,  experiments,  comparisons  and 
conclusions.  The  notes  may  be  kept  on  detached  sheets  sim- 
ilar to  those  used  for  the  drawings,  if  desired. 

Outline  drawings  and  diagrams  must  be  made  of  every 
structure  or  relation  which  can  be  shown  by  a  well-labeled 
sketch.  Shading  should  be  sparingly  used  and  only  with  a 
matured  purpose,  the  result  first  being  tested  on  a  separate 
sheet  of  paper.  The  name  of  each  portion  of  the  sketch  should 
be  determined  and  named  by  running  a  leader  from  the  part 
to  an  appropriate  place  for  the  name.  The  drawings  are  al- 
ways to  be  made  in  the  laboratory  and  from  the  specimen 
studied.  It  is  through  the  judicious  criticism  of  the  drawings 
that  the  teacher  can  best  bring  out  the  deficiencies  in  the  stu- 
dent's observations.  One  teacher  cannot  do  justice  to  a  labor- 
atory section  of  more  than  ten  or  twelve  students.  A  good 
portion  of  the  failure  accredited  in  some  quarters  to  the  labora- 
tory method  is  due  to  inefficient  direction.  The  laboratory 
will  no  more  run  itself  than  will  the  class  room. 

It  is  very  desirable  that  students  keep  a  field  note-book,  of 
size  suitable  for  the  pocket,  in  which  all  his  own  observations 
should  be  entered  and  dated.  These  notes  may  be  put  into 
fuller  form  in  the  reports  called  for  in  the  body  of  the  text. 
It  is  chiefly  through  the  encouragement  of  such  work  as  this 
that  the  teacher  may  hope  to  develop  in  his  students  a  perma- 
nent interest  in  natural  history,  which  will  contribute  mate- 
rially to  their  satisfaction  in  later  life.  It  is  thus  that  men  and 
women  come  to  devote  their  lives  to  nature  study. 


SUGGESTIONS    TO    TEACHERS.  465 

7.  Library  Helps. — The  library  is  quite  as  necessary  to  a 
balanced  course  of  zoology  as  the  text-book,  the  teacher,  or 
the  laboratory.  First  under  this  head  may  be  considered 
charts.  The  teacher  should  become  as  expert  as  possible  in 
making  diagrams  on  the  board  before  the  eyes  of  the  pupils. 
These  may  be  supplemented  by  charts  made  by  the  teacher,  or 
the  pupils,  by  enlarging  figures  found  in  the  text-books.  Such 
diagrams  have  a  distinct  advantage  over  the  originals  in  that 
they  may  be  discussed  while  in  view  of  the  whole  class.  For 
this  purpose  a  good  quality  of  light-colored  wrapping  paper 
will  serve,  if  better  drawing  paper  cannot  be  had.  Keuffel 
and  Esser  (New  York  and  Chicago)  will  send  samples  of 
drawing  paper  on  application.  The  outlines  should  be  made 
in  water-colors  with  a  suitable  brush,  in  lines  heavy  enough  to 
be  clearly  visible  across  the  room.  Colors  may  be  put  on  with 
crayon  and  fixed  by  a  spray  of  shellac. 

Photographs  and  lantern  slides  are  of  value  in  illustrating 
the  structure,  development,  and  habits  of  animals.  If  the 
school  can  command  a  lantern  or  a  heliopticon,  a  collection  of 
lantern  slides,  selected  in  accordance  with  the  special  interests 
of  the  teacher  and  pupils,  becomes  a  great  stimulus  in  natural 
history  work.  If  a  large  collection  of  books  is  impossible  the 
brief  lists  below  will  assist  the  teacher  in  selecting  the  most 
helpful  reference  books  for  an  elementary  course.  More  ex- 
tended bibliographical  lists  will  be  found  in  many  of  the  books 
cited.  In  a  general  way  those  considered  most  essential  are 
placed  first  under  the  main  headings.  A  very  good  working 
collection  of  books  may  be  secured  for  about  $150  to  $200. 

In  every  written  report  demanding  library  work  it  is  desira- 
ble to  have  the  student  record  in  his  paper  a  list  of  all  the 
references  bearing  on  the  subject.  It  is  customary  to  arrange 
the  authorities  alphabetically,  together  with  such  other  facts 
as  are  needed  for  ready  reference.  The  following  illustration 
will  serve  to  indicate  what  facts  should  be  recorded : 
31 


466  ZOOLOGY. 

Parker  and  Haswell, 

'97.  A  Text-Book  of  Zoology.  Vol.  i,  pp.  580-583: 
illustrations. 

In  addition  to  the  books  listed  below  the  teacher  should 
endeavor  to  secure,  through  his  representative  in  congress, 
the  publications  of  the  U.  S.  Department  of  Agriculture: 
the  Yearbook,  the  Farmers'  Bulletins,  the  bulletins  of  the 
Bureau  of  Animal  Industry  and  of  the  Division  of  Entomol- 
ogy. The  Reports  and  Bulletins  of  the  U.  S.  Fish  Commis- 
sion contain  much  valuable  material.  The  publications  of  the 
state  surveys  and  of  the  experiment  stations  of  the  various 
states  are  often  of  very  high  value  to  the  teacher  and  are 
usually  distributed  gratuitously. 

It  will  be  helpful  to  make  as  large  a  collection  as  possible 
of  the  current  elementary  texts  of  zoology,  and  adopt  the  best 
suggestions  of  each. 

BIBLIOGRAPHY. 
I.. GENERAL:  ANATOMY,  EMBRYOLOGY,  PHYSIOLOGY,  ECOLOGY,  HABITS,  ETC 

1.  Thompson,  J.  A.     Outlines  of  Zoology.     Third  edition.      1899.      D. 

Appleton  and  Co.,  New  York     Price  $4.50. 

2.  Parker  and   Haswell.    Text-book   of   Zoology.    2  vols.     1897.     The 

Macmillan  Co.,  New  York.     Price  $9. 

3.  Hertwig,    R.     General    Principles   of   Zoology.     Translated   by   Field, 

1896.     Henry  Holt  and  Co.,  New  York.     Price  $1.60. 

4.  Semper,  K.     Animal  Life  as  Affected  by  the  Natural  Conditions  of 

Existence.     1881.     D.    Appleton    and    Co.    (Int.    Sci.    Series),    New 
York.     Price  $2. 

5.  Jordan,  D.  S.    Animal  Life.     1900.     D.  Appleton  and  Co.,  New  York. 

Price  $1.20. 

6.  Verworn,  M.    General  Physiology;  An  Outline  of  the  Science  of  Life. 

Translation,  1899.     The  Macmillan  Co.,  New  York.     Price  $4. 

7.  Mills,  W.    Text-book  of  Animal  Physiology.     1889.     D.  Appleton  and 

Co.,  New  York.     Price  $4. 

8.  Wilson,   E.   B.    The   Cell  in  Development  and   Inheritance.     Second 

Edition.     1900.     The  Macmillan  Co.,  New  York.     Price  $3. 

9.  Wallace,  A.  R.    Geographical  Distribution  of  Animals.    2  vols.     1879. 

Harpers,  New  York.     Price  $10. 

10.  Haddon,    A.    C.    Introduction   to    the    Study   of   Embryology.     1887. 

P.  Blackiston  and  Co.,  Philadelphia. 

11.  Morgan,   T.   H.    The   Development   of   the   Frog.     1897.    The   Mac- 

millan Co.,  New  York.     Price  $1.60. 


SUGGESTIONS    TO    TEACHERS.  467 

12.  Bulletin  U.  S.  National  Museum,  No.  39  (parts  A  to  M  thus  far  pub- 

lished).    Directions  for  collecting  and  preserving  various  kinds  of 
animals.     Washington,  D.  C.     Price,  a  few  cents  for  each  part. 

13.  Marshall    and    Hurst.      Practical    Zoology.      1888.      G.    P.    Putnam's 

Sons,  New  York ;  Smith,  Elder  and  Co.,  London.     Price  $3.50. 

14.  Lubbock,  J.     On  the   Senses,   Instincts,  and   Intelligence   of  Animals. 

(Int.  Sci.  Ser.)     D.  Appleton  and  Co.,  New  York,  1888.     Price  $1.75. 

15.  Hornaday,  W.  T.     Taxidermy  and  Zoological  Collecting.     1897.    Chas. 

Scribner's  Sons,  New  York.     Price  $2.50. 

16.  Morgan,  C.  L.     Animal  Life  and  Intelligence.     1891.     Ginn  and  Co., 

Boston.     Price  $4.      (Out  of  print.) 

17.  Morgan,    C.    L.     Habit    and    Instinct.     1896.     Edward    Arnold,    New 

York.     Price  $4. 

18.  Poulton,  E.  B.    The  Colors  of  Animals.     (Int.  Sci.  Ser.)     1890.     D. 

Appleton  and  Co.,  New  York.     Price  $1.75. 

19.  Wallace,  A.  R.    Tropical  Nature.     1895.     The  Macmillan  Co.     Price 

$1-75- 

20.  Darwin,  Chas.     The  Variation  of  Animals  and  Plants  under  Domesti- 

cation.    2  vols.     1875.    'D.  Appleton  and  Co.,  New  York.     Price  $5. 

21.  Le  Conte,  J.     Outline  of  the  Comparative  Physiology  and  Morphology 

of  Animals.     1900.     D.  Appleton  and  Co.,  New  York.     Price  $1.80. 

22.  Clark,  C.  H.     Practical  Methods  in  Microscopy.     1896.     D.  C.  Heath 

and  Co.,  Boston.     Price  $1.60. 

23.  Beddard,   F.   E.     A   Text-book   of   Zoogeography.     1895.     Cambridge 

(Eng.)  Univ.  Press.     Price  $1.50. 

24.  Davenport,  C.  B.    Experimental  Morphology,  Part  I,  1897;   Part  2, 

1899.     The  Macmillan  Co.,  New  York.     Price  $4. 

25.  Eimer,  G.  H.  T.     Organic  Evolution.    1890.    The  Macmillan  Co.,  New 

York.     Price  $4. 

26.  Shafer,  E.  A.     Essentials  of  Histology.     Fourth  Edition.     1894.     Lea 

Bros.,  Philadelphia. 

27.  Shipley,  A.  E.    Zoology  of  the  Invertebrates.     1893.     A.  and  C.  Black, 

London.     Price  $6.25. 

28.  Van    Beneden,    F.     Animal     Parasites    and   Messmates.      1876.      D. 

Appleton  and  Co.,  New  York.     Price  $1.50. 

29.  Schmeil,  Otto.    Text-book  of  Zoology,  from  a  Biological  Standpoint. 

1901.     A.    and    C.    Black,    London.     Price   $4. 

30.  Hodge,  C.  F.     Nature  Study  and  Life.     1902.     Ginn  and  Co.,  Boston. 

31.  Scott,  Chas.  B.     Nature   Study  and  the  Child.     1902.     D.   C.   Heath 

and  Co.,  Boston. 

32.  Riverside  Natural  History;  J.  S.  Kingsley,  Editor.     6  vols.    Houghton, 

Mifflin  and  Co.,  Boston.     Price  $30. 


468  ZOOLOGY. 

II.  SPECIAL:  BOOKS  TREATING  THE  STRUCTURE  AND  CLASSIFICATION  OF  THE 
MORE  IMPORTANT  GROUPS  OF  ANIMALS. 

1.  Jordan,   D.   S.     Manual   of  the  Vertebrates   of  the    Northern   United 

States.     Eighth   Edition.     1899.     A.   C.    McClurg  and   Co.,    Chicago. 
Price  $2.50. 

2.  Chapman,  F.  M.     Bird  Life.     A  Guide  to  the  Study  of  Our  Common 

Birds.     1897.     D.  Appleton  and  Co.,  New  York.     Price  $2. 

3.  Chapman,   F.   M.     Hand-book   of   Birds   of   Eastern   North   America. 

1900.     D.  Appleton  and  Co.,  New  York.     Price  $3. 

4.  Comstock,  J.  H.     Manual  for  the  Study  of  Insects.     1895.     Comstock 

Publishing  Co.,  Ithaca,  New  York.     Price  $3.75. 

5.  Folsom,  J.  W.     Entomology :  with  special  reference  to  its  Biological 

and  Economic  Aspects.     1906.     P.  Blakiston's  Son  and  Co.     Phila- 
delphia.    Price  $3. 

6.  Lubbock,    J.    Ants,    Bees    and    Wasps.     Int.    Sci.    Series.     1882.     D. 

Appleton  and  Co.,  New  York.     Price  $2. 

7.  Emerton,  J.   H.     Common   Spiders.      1902.      Ginn   and   Co.,   Boston, 

Mass.     Price  $1.50. 

8.  Herrick,  F.  H.    The  American  Lobster:  Its  habits  and  development. 

1896.     Bulletin  United   States  Fish   Commission,   Vol.   XV. 

9.  Darwin,    Chas.    Vegetable    Mould    and    Earthworms.      D.    Appleton 

and  Co.,  New  York.     Price  $2. 

10.  Calkins,   Gary  N.    The   Protozoa.     1901.     The   Macmillan   Co.,   New 

York.     Price  $3. 

11.  Howard,  L.  O.     Mosquitoes.     1901.     McClure,  Phillips  and  Co.,  New 

York.     Price  $1.50. 

12.  Baskett,  J.  N.    The  Story  of  the  Birds.     1899.     D.  Appleton  and  Co., 

New  York.     Price   65   cents. 

13.  Cowan,  T.  W.     Natural  History  of  the  Honey  Bee.     1890.     Houston, 

London,     is.    6d. 

14.  Holland,  W.  J.    The  Butterfly  Book.     1899.     Doubleday  and  McClure 

Co.,  New  York.     Price  $3. 

15.  Holland,   W.  J.     The   Moth   Book.      1904.     Doubleday   and    McClure 

Co.,  New  York.     Price  $3. 

16.  Comstock,   J.    H.     Insect   Life.     1901.     D.    Appleton   and    Co.     Price 

$1.50. 

8.  Collections. — While  the  educative  value  of  a  miscel- 
laneous assortment  of  the  curios  so  often  brought  to  teachers 
is  not  great,  a  permanent  collection  of  the  typical  animals  of 
the  locality  may  be  so  arranged  as  to  be  of  considerable  value 
for  comparison.  There  should  be  added  to  these  gradually, 
by  purchase  or  otherwise,  representatives  of  those  classes  of 
animals  not  represented  in  the  local  fauna  in  order  to  give  the 


SUGGESTIONS    TO    TEACHERS.  469 

collection  more  of  a  synoptic  value.  Such  a  collection  of  types 
from  the  more  important  classes  of  animals  serves  an  impor- 
tant end  in  giving  the  student  a  general  idea  of  the  animal 
kingdom  as  a  whole,  which  is  difficult  to  gain  in  any  other  way. 
The  building  up  of  a  laboratory  museum  with  the  help  of  the 
students  may  be  made  to  serve  as  an  incentive  to  care  and  neat- 
ness on  their  part  in  making  dissections  or  other  preparations 
which  may  bear  the  name  of  the  student  on  the  labels,  when 
permanently  installed  in  the  collection. 

It  is  not  amiss  to  encourage  students  in  the  special  study 
of  some  limited  group  of  animals,  and  this  may  frequently  be 
accomplished  by  the  beginnings  of  a  collection  of  the  local 
species  of  the  group.  The  permanent  interest  and  enthusiasm 
on  the  part  of  the  pupil  in  the  study  of  the  phenomena  of  liv- 
ing things  may  be  taken  as  the  measure  of  success  in  teaching 
the  natural  history  sciences.  This  can  be  secured  more  readily 
by  studying  life  in  its  natural  surroundings  than  by  dissections. 


INDEX. 


[The  names  of  the  genera  cited  are  printed  in  italics.  The  numerals  refer 
to  pages.  Those  in  bold-faced  type  indicate  that  the  object  is  illustrated  on 
that  page.] 


Absorption,  61 

Abyssal   fauna,    129 

Acalephs,  242 

Acanthopteri,  369 

Acclimatization,    12,    98 

Achromatin,   20,   26 

Acquired   characters,    100 

Actinozoa,   177 

Adaptation,  means  of,  98 ;  types  of, 
104;  to  inorganic  environment,  105  ; 
to  mates,  107;  to  offspring,  108; 
social,  112,  113;  competitive,  114; 
commensal,  116;  symbiotic,  116;  to 
preying,  118;  for  protection,  119 

Adaptive  habits  and  instincts,   126 

Adipose  tissue,  48 

Aepyornis,    409 

Aerial  fauna,  130. 

Air-sacs   (birds),  401 

Aix,  408 

Albatross,  411 

Alimentary  canal  (see  Digestive  sys- 
tem). 

Allantois,  387,  405 

Alligator,  392 

Allolobophora,  216 

Alternation  of  generation — in  Coelen- 
terates,  172;  in  worms,  189,  190 

Alveoli,   436 

Ambulacral  system,  207,  210 

Amitotic  cell  division,  23 

Ammonites,    259 

Amnion,  387,  405 

Amoeba,  1.3,  22,  84,  142,  145 

Amphibiarfs,    137,   372 

Amp hioxus,  309,  jig^ 

Amphitrite,  231 

Amphiuma,   377 

Ampullae,  207,  210 

Anabolism,  60 

Anacanthini,  369 

Analogy,   78 

Anaphase,  23 

Anatomy,  3 

Animalcules   (see  Protozoa) 


Annelids    (see   Annulata) 

Annulata,  139,  216, 

Anodonta,  234,  253 

Ant-eater,  439,  441 

Antenna,   263 

Antennule,  263 

Antero-posterior   (see   symmetry) 

Antimeres,   87 

Ants,  114,  303 

Anura,  379 

Aorta,  337  (see  circulatory  system) 

Apes,  447 

Aphis   (plant  lice),  296 

Appendages — arrangement   of,   88  ;   in 

arthropods,  275  ;  in  vertebrates,  322  ; 

in  fishes,  356  ;  in  amphibians,  373  ; 

in   birds,   397 ;   in   mammals,   429 
Appendicular  skeleton,  328,  329 
Appendix,  455 
Aptera,  293 
Apteryx,  406 
Aquatic   fauna,    129 
Arachnida,  303 
Araneida,  304 
Arbacia,  214 
Archceopteryx,  406 
Archenteron,  33,  35 
Archipterygium,  357 
Ardea,  410 
Argonauta,  258 
Aristotle's  lantern,  203,  207 
Armadillo,   441 
Army-worm,   300 
Arterial  arches,  337,  338 
Arteries,  64    (see  circulatory  system) 
Arthropoda,   138,  261 
Artificial  selection,  97 
Artiodactyla,    442 
Ascaris,    193 
Ascidians,  309 
Asellus,  265 
Ass,  443 

Assimilation,   12   (see  metabolism) 
Asterias,   200 
Asteroidea,  213 
Astroid-stage,  23    (cell-division) 


471 


472 


INDEX. 


Asymmetry,  84,  368 

Atrial  pore,  310 

Atrium,   310 

Auditory   nerve,    349 

Auditory  organs,  75,  281,  350 

Auk,    411 

Aurelia,  176,  177 

Autolytus,  229,  230 

Automatism,   n 

Aves.   394 

Axiality,  83,  84 

Axial  skeleton,   325 

Axon,  55 

Baboons,  447 
Balanoglosstts,  308,  309 
Barbs  (see  feather) 
Barbules  (see  feather) 
Barnacles,   288 
Bass   (sea),  367 
Bats,  446 
Beaver,  444 
Bees,   114,  303 
Beetles,  301 
Bibliography,  466 
Bilateral  symmetry,  86,  88 
Birds,  135 
Bittern,  412 
Bivium,  205 
Blastoidea,   213 
Blastomeres,  32 
Blastopore,    35 
Blastula,  34 
Blood,  51,  335 
Boat-shell,    255 
Body-wall  (see  Ccelom) 
Bojanus,  organ  of  (—  kidney  of  clam) 
Bonasa,  413 
Bone,  49 
Bony-fishes,  369 
Book-lungs,   278 
Bat-fly,  296,  297 
Bothriocephalus,  198,  199 
Brachiopoda,  196 
Brain    (vertebrate),   345 
Branchiae,    63,   207 

Branchial    arches,    337-339    (see    cir- 
culatory system) 
Brittle-stars,  214 
Bronchi,  436 
Brook  trout,   365 
Bryozoa    (see   Polyzoa,    195) 
Buccal  cavity,   331 
Bugs,  296 
Bubo,  416 
Bufo,  378 
Butterflies,  300 


Cabbage   butterfly,    299 

Cabbage  worm,  299 

Caeca,  hepatic,  207 

Caecum,  434 

Cainbarus,  261 

Camel,  442 

Camera  eye.   76,   352 

Campanularia,    177 

Campodea,  294 

Canaliculi,  49 

Canine  teeth,  432 

Cankef-worm,  300 

Capillaries,  65 

Caprella,  292 

Carapace — in   Crustacea,  ,279  ;    turtles, 
388 

Carbohydrates,    10 

Cardinal    veins,    362    (see    circulatory 
system) 

Cardium,    253 

Carinatae,  410 

Carnivora,   443 

Cartilage,  48 

Cat,  443 

Caudal   vertebrae,   327 

Cecidiomya,  298 

Cell,   1 8 

form,  18;  size,  19;  structure,  19; 
wall,  20 ;  functions,  21  ;  reproduc- 
tion, 22  ;  division,  22 

Centipede,  293 

Central  nervous  system,  71,  344 

Centropristis,  367 

Centrosome    (centrosphere),    20 

Centrum   (see  vertebra),  325 

Cephalopoda,  257 

Cephalo-thorax,    262,   275 

Ceratodus,  370 

Cercaria,  189,  190 

Cervical  vertebrae,  327 

Ceryle,  419 

Cestodes,  ^9,9 

Cetacea,  441 

Chaetopoda,  230 

Chaetognatha,    233 

Chameleon,   388 

Cheiroptera,    446 

Chelonia,  388 

Chemical    sense,    74,    350    (see    taste 
and  smell) 

Chicken,  413 

Chimney-swifts,  422 

Chlamydomyxa,  20 

Chordata,  308   (see  vertebrata) 

Chorion,  405,  438 

Chylema,  20 

Chromatin,  20,  26 

Cicada,  295,  296 


INDEX. 


473 


Cilia,  21,  43,  147 

Circulation,  61,  64 

Circulatory  organs,  64 ;  in  Echino- 
derms,  209 ;  in  annulata,  225  ;  in 
mollusks,  245,  246 ;  in  arthropods, 
279;  in  vertebrates,  335-340  ;  in 
fishes,  337,  361,  362  ;  in  amphibia, 
338,  3755  in  reptiles,  339,  384;  in 
birds,  340  ;  in  mammals,  339,  435 

Cirratulus,   232 

Cirripedia,  288 

Cistudo,  388 

Clam,  234 

Clammatores,  422 

Classification — defined,  5;  use,  100; 
illustration  of,  101 

Classification — protozoa,  153;  pori- 
fera,  163;  coelenterata,  177;  un- 
segmented  worms,  185  ;  annulata, 
230 ;  mollusca,  252 ;  arthropoda, 
288  ;  vertebrata,  354  ;  fishes,  368  ; 
amphibia,  379  ;  reptiles,  388  ;  birds, 
407  ;  mammals,  439 

Cleavage,  32 

Clitellum,   217 

Cloaca,   159,   333 

Clothes-moth,  300 

Clypeus,  269 

Coccyzus,  420 

Cockroaches,  294 

Cocoon,  228,  285 

Ccelenterata,   139,  165 

Coelom  (defined),  36;  in  unsegmented 
worms,  184;  echinoderms,  207; 
annulata,  223  ;  arthropods,  276 ; 
vertebrates,  322 

Ccelomata   and  coelenterata,    139 

Coil  stage  (cell-division),  24 

Coleoptera,  301 

Colonies — organic,  112;  voluntary,  113 

Color,    120,   367 

Colpoda,  20 

Columba,  416 

Columbae,  417 

Commensalism,   116 

Compound  eye,  78,  281,  282 

Condyle,  383,  396 

Conjugation,  67 

Congo-snake,   379 

Connective  tissue,  46 

Contractility,  14   (see  motion) 

Convolutions    (brain),  436 

Coordination,   71 

Corallines,  195 

Corallum,  174 

Corpuscles  (blood),  51 

Cotton-boll  worm,  300 

Coverts    (see   feather) 


Coxa,  269 

Crab,  286,  288;  larvae,  284,  285 

Crane,  412 

Cranial  nerves,   348 

Crayfish,  261,  288 

Crepidula,  255 

Crickets,  294 

Crinoidea,  213 

Crocodiles,   392 

Cro talus,  383 

Crows,  422 

Crustacea,   288 

Ctenophora,   175,   177 

Cuckoo,  419 

Cursores,  408 

Cyclas,  253 

Cyclops,  266,  268 

Cyclostomes,  311 

Cyrtophyllus,  295 

Cyst,  151 

Cystoidea,  213 

Cytoplasm,  19,  25 

Daphnia,  266,  268 

Decapoda,   259,   289 

Deer,  442 

Degeneration,  125 

Dendron,  55 

Dental   formula,   433 

Dentine,  50,  432  (teeth) 

Dermis,  323   (see  integument) 

Dermo-muscular  sac,   222 

Dero,  212,  220,  232 

Derotremata,  3^ 

Development  (see  metamorphosis) — 
porifera,  163;  coelenterata,  175;  un- 
segmented worms,  1 88,  189,  190 ; 
echinoderms,  211;  annulata,  227; 
mollusks,  249 ;  arthropods,  283 ; 
vertebrates,  342,  343 ;  fishes,  366 ; 
amphibians,  376 ;  reptiles,  387 ; 
birds,  403  ;  mammals,  437 

Devil-fish,  257,  258 

Diastroid  stage,  23 

Dibranchiata,  259 

Didelphia,  44° 

Didelphys,    106 

Differentiation  of  tissues  and  organs, 
27,  37,  40 

Digestion,  60,   61 

Digestive  system  —  protozoa,  147; 
sponges,  161  ;  coelenterata,  172;  un- 
segmented worms,  1 86,  187;  echi- 
noderms, 206  ;  annulata,  224  ;  mol- 
lusks, 243  ;  arthropoda,  276  ;  verte- 
brates, 330  ;  birds,  400  ;  mammals, 
433  ;  ruminants,  434 

Digestive  tract,  regions,  331 


474 


INDEX. 


Dimorphism    (sexual),    69 

Dipnoi,  369 

Diptera,  296 

Direct  cell-division,  23 

Discophora,  232   (leeches) 

Dispersal  of  animals,  128 

Dissimilation,   15   (katabolism) 

Distomnm,  189,  190 

Distribution,  geographical,  4,  129 

Division    (cell),    23 

Division  of  labor,  59 

Dodo,  416 

Dogs,  443 

Domestication  of  animals,  97 

Dorsal  vertebrae,  327 

Dorso-ventral  axis   (see  symmetry) 

Dove,  416 

Draco,  390 

Duck-bill,  439 

Ducks,  408 

Ductus  Cuvieri,  362 

Dugong,  441 

Eagle,  417 

Ear,   350    (see  auditory   organs) 

Earthworm,   216 

Echidna,  439 

Echinodermata,    138,   200 

Echinoidea,   214 

Echinus,  203 

Ecology — defined,  4  (see  also  adapta- 
tion) ;  of  protozoa,  146,  154  ;  porifera, 
163;  coelenterata,  178;  unsegmented 
worms,  197,  199;  echinoderms,  212; 
annulata,  230  ;  mollusks,  251  ;  ar- 
thropods, 286 :  fishes,  363,  367 ; 
amphibia,  376  ;  reptiles,  385  ;  birds, 
403  ;  mammals,  448 

Ectoderm,    34 

Ectosarc,   145 

Edentata,  441 

Egg,  29 

Egret,  412 

Elasmobranchii,  368 

Electric  organs,  367 

Elephant,  443 

Elytra,  301 

Embryology,  defined,  4  (see  develop- 
ment) 

Embryonic  tissues,  50 

Emu,  408 

Enamel,  50  (see  teeth),  432 

Endopodite,  263 

Endosarc,    145 

Endoskeleton,  67,  324,  399  (see  also 
skeleton) 

Endothelium,  42 

Ensis,  253 


Entoderm,  34 

Entomostraca,  288 

Environment,   92 

Epcira,   305 

Epiblast    (see  ectoderm) 

Epicranium,  269 

Epidermis  (see  integument),  323 

Epinephelus,  370 

Epithelial  tissue,   42-46 

Equilibrium  sense,  75,  349,  351 

Erethizon,  445 

Euglena,  144,  150 

Eutccnia,  391 

Euthyneura,  256 

Eustachian  tube,  351 

Excretion,   16,  61,  65 

Excretory  systems — coelenterates,  174 
unsegmented  worms,  186,  188 
echinoderms,  209 ;  annulata,  225 
mollusks,  247 ;  arthropods,  280 
vertebrates,  341 

Exopodite,  263 

Exoskeleton,  67,  324   (see  also  integ- 
ument) 

Eyes,     76-78;     281,     282,     351,     352; 
median,    385 

Eye-spots,    ccelenterata,    175;    echino- 
derms, 2ii  ;  annulata,  226 

Facial  nerve,   349 

False  amnion,   405 

Fat  cells,  48 

Fats,   10 

Faunas,  type  of,  129 

Feathers — kinds,   395,   399  ;•  structure, 

395 

Femur,  269 
Ferments,  10 
Fertilization,  30,  68 
Fibrillae   (muscle),  53 
Fibrous   connective   tissue,  48 
Fin,  357 

Fishes,  137,  312,  355 
Fish  Commission,  work  of,  366 
Fission,  184,  188,  229 
Flame-cell,  186,  187 
Flamingo,  411 

Flat-worms  (see  Platyhelminthes,  185) 
Fleas,  297 
Flies,  297 
Foetal    membranes    (see    amnion    and 

allantois) 
Foraminifera,  153 
Formative  pole,  32 
Fox,  443 
Frog,  315,  378 
Frontal  section,  83 
Fruit-borers,    300 


INDEX. 


475 


Functions    (basal),    60 
Fur  (see  hair) 

Galea,  269 

Gallinse,  412 

Callus,  413 

Gamete,   151 

Gammarus,  291 

Ganglion,   55 

Ganoidei,   368 

Gar-pike,   368 

Gasteropoda,  254 

Gastrophilus,  296,  297 

Gastro-vascular  cavity,   170,   171 

Gastrula,  33,  34 

Gavials,    392 

Geese,  411 

Gelatinous  tissue,  48 

Genae,  269 

Genus,    102 

Germinal  layers,  35 

Germinative  epithelium,  341 

General  characters — protozoa,  145  ; 
porifera,  157;  ccelenterata,  170;  un- 
segmented  worms,  183  ;  echino- 
derms,  203  ;  annulata,  219 ;  mol- 
lusks,  239  ;  arthropods,  274 ;  chor- 
data,  308  ;  vertebrates,  321  ;  fishes, 
355  ;  amphibia,  372  ;  reptiles,  381  ; 
birds,  396 ;  mammals,  428 

General  survey — protozoa,  144;  pori- 
fera, 157  ;  159  ;  ccelenterata,  168, 
170;  echinoderms,  203;  unseg- 
mented  worms,  183  ;  annulata,  221  ; 
mollusks,  240  ;  arthropods,  275  ; 
vertebrates,  ch.  XIX ;  fishes,  355 ; 
amphibia,  372  ;  reptiles,  380  ;  birds, 
396  ;  mammals,  429 

Gila-monster,    388 

Gill-arches,    334 

Gills,  63,  334 

Girdles,  328,  329 

Gizzard,  277,  332,  333 

Glands,  42;  in  mammalia,  324,  431 

Glass-snake,  388 

Glosso-pharyngeal  nerve,  349 

Glottis,  335 

Gnats,  297 

Goblet  cells,  43 

Goldfinch,  422 

Gonads,   175   (see  ovary  and  testis) 

Grallatores,  411 

Grantia,  156 

Grasshopper,  266 

Green  gland,  264,  280 

Grouse,  413 

Growth,   13,  67 

Guinea-fowl,  413 


Gulls,  411,  409 
Gymnophiona,   379 

Habits  and  habitats  (see  ecology) 

Hsemocele,  276 

Haemoglobin,  336 

Hair,   431 

Hair-follicle,   430 

Hares,  444 

Harvest-men,  306 

Hawks,   417 

Haversian  canal,  49 

Hearing,  75  (see  ear) 

Heart,  336  (see  circulatory  system) 

Hedge-hog,  446 

Helix,  241,  256 

Heloderma,  390 

Hemiptera,  296 

Hepatic  portal  circulation,  362 

Heredity,   92  ;   carriers  of,  93 

Hermaphroditism,   69,    163 

Hermaphrodite  *  organs  —  tapeworm, 

192  ;   snail,   249 
Hermit  crab,    116,   178  s 
Heron,  410,  412 
Hessian  fly,   298 
Heterocercal  tail,  358 
Heteromya,  254 
hexapoda,  293 
Hippopotamus,  442 
Historical — general,   6;   protozoa,   151 
Histology — defined,  3 
Hog,  442 

Holothuroidea,  214^ 
Homarus,  289 
Homocercal  tail,  358 
Homology,  78 
Horned-toad,   388 
Hornbill,  419 
Horse,  442 
Horse-mussel,  254 
Humming-bird,  422 
Hybrid,  102 
Hydra,  165,  169 
Hydractina,  178 
Hydroid  type,  170 
Hydrozoa,   177 
Hylocichla,  423 
Hymenoptera,   303 
Hypoblast,  34  (see  entoderm) 

Ichneumon  fly,  303 
Imago,  285 
Incisor  teeth,  432 
Infusoria,  154 
Ingestion  of  food,  61 
Insectivora,  445 
Insects,  293 


476 


INDEX. 


Integument  (see  skin) — annulata,  222  ; 
echinoderms,  238 ;  mollusks,  242 ; 
arthropods,  276  ;  vertebrates,  322  ; 
fish,  359 ;  amphibians,  373  ;  rep- 
tiles, 382  ;  birds,  398  ;  mammals,  431 

Intercellular  substance,  41 

Intestine,  333  (see  digestive  system) 

Invagination,   34 

Irritability,   n,  70   (see  sensation) 

Jackals,  443 

Jelly-fish,   171   (see  medusa) 

Jugular  vein,  362 

Katabolism,   15,  60 

Katydid,  249,  295 

Keeled  birds,  410 

Kidney,  66,  341   (see  nephridia) 

King  crab,  303 

Kingfisher,  419 

Laboratory  exercises — protozoa,  142  ; 
porifera,  156;  coelenterata,  165;  un- 
segmented  worms,  185 ;  echino- 
derms, 200;  annulata,  216;  mol- 
lusks, 234  ;  arthropods,  261  ;  chor- 
data,  312  ;  fish,  312  ;  amphibian,  315  ; 
reptile,  380  ;  birds,  394 ;  mammals, 
427 

Labium,  269 

Labrum,  269 

Lacertilia,  388 

Lacinia,  269 

Lacteals,  436 

Lacunae — bone,  49,  50;  blood,  280 

Ladybird  (ladybug)  beetle,  302 

Lamella  (bone),  49,  50 

Lamellibranchiata,  252 

Lamprey,  126,  311 

Lampropeltis,  391 

Lancelet,   309,  310 

Lanius,  425 

Lark,  424 

Larynx,  335 

Leech,  219,  232 

Leopard,  443 

Lemurs,  447 

Lepidoptera,   300 

Lepidosiren,   370 

Lepornis,  369 

Leucania,  300 

Lice,  296 

Life — nature  of,  8  ;  relation  to  pro- 
toplasm, 8 

Limax,  256,  257 

Limbs,  bones  of,  329 

Limntza  (host  of  liver  fluke,  190).  237 

Limpet,  255,  256 


Liuiulus,  278,  303 

Lion,  443 

Littoral  fauna,  129 

Littorina,  255 

Liver-fluke,  188,  189 

Lizards,  388 

Lobster,  289,  290 

Locomotion,  69 

Locomotor  organs — protozoa,  148  ; 
porifera,  161  ;  coelenterata,  174;  un- 
segmented  worms,  197  ;  echino- 
derms, 207,  210;  annulata,  222 ; 
mollusks,  243  ;  arthropods,  276 ; 
fishes,  360  ;  amphibia,  375  ;  reptiles, 
382  ;  birds,  397  ;  mammals,  428 

Locust-borers,  302 

Locusts,  294 

Lophobranchii,  369 

Lophophore,   195 

Lumbar  vertebrae,  327 

Lumbrictis,  216 

Lung-fishes,  369 

Lungs,  63,  335 

Lymph,  51 

Lymphatics,  435 

Macromere,  227,  249 

Madreporic  body,  207,  210 

Malacostraca,  289 

Malpighian  tubule,  278 

Mammals,   135 

Mammary  glands,  324,  431 

Mammoth,   443 

Man,  447 

Manatee,  441 

Mandible,  263,  269 

Mantle,  242 

Marsupials,  429,  440 

Marsupium,  429 

Marten,  443 

Mastodon,  443 

Maturation  (of  ovum),  30,  y^ 

Maxilla,  263,  269 

Maxillary  palpus,  269 

Maxillipeds,  263,  269 

May-flies,  296 

Median  eye,   385 

Medullary  sheath  (nerve),  54,  55 

Medusa-type,    171. 

Megalops,  285 

Mesenchyma,  158   (mesogloea) 

Mesenteron,  60,  277,  330 

Mesentery,   314 

Mesoblast  (see  mesoderm) 

Mesoderm,  35 

Mesothorax,  269 

Metabolism,   15,  60 

Metamere,  87,  89 


INDEX. 


477 


Metamorphosis — worms,  189,  190; 
echinoderms,  212;  arthropods,  284; 
amphibia,  376,  377 

Metaphase,  23 

Metapleure,  310 

Metathorax,  269 

Metazoa,   140 

Metridium,  167 

Mice,  444 

Micromeres,  227,  249 

Micropyle,   30 

Micro  stoinum,  188 

Migration,  98,  128  ;  of  birds,  425 

Millipedes,  293 

Mimicry,  122,  386 

Mimus,  423 

Mink,  443 

Mites,  306 

Mitotic  cell-division,   23 

Mocking  bird,  423 

Modern  birds,  407 

Modiola,  254 

Molar  teeth,  432 

Moles,  446 

Mollusca,   138 

Molluscoidea,    195 

Mongrels,   102 

Monkeys,  447 

Monodelphia,  440 

Monomya,  254 

Monotremata,  429,  439 

Morphology — denned,  3 

Morula,  34 

Mosquitoes,  296,  298 

Moths,  300 

Motion,  69  (see  locomotion)  ;  proto- 
plasmic, 14 

Moulting,  276,  399 

Mouth-parts  (arthropods),  263,  269 

Mucous  epithelium,  42 

Muscular  system — coelenterates,  1 74  ; 
echinoderms,  209 ;  annulates,  222  : 
mollusks,  243 ;  arthropods,  277 ; 
vertebrates,  342 

Muscular  tissue,  52-55 

Mussel,  234,  252,  254 

Mya,  234,  252 

Myctodera,  379 

Myotome,  310,  313,  344 

Myriapoda,  293 

Mytilus,  254 

Nais,  232 

Natatores,  411 

Natural  selection,  97 

Nauplius,   289 

Nautilus — pearly,  257;  paper,  258 

Necturus,  377,  379 


Nemathelminthes,   193 

Neornithes,  407 

Nephridia,  66,  225,  341 

Nereis,  219,  230 

Nervous  system,  71-78  (see  sense  or- 
gans) ;  coelenterata,  175;  echino- 
derms, 210  ;  annulata,  226  ;  mollusks, 
247  ;  arthropods,  280  ;  vertebrates, 
344  ;  reptiles,  385  ;  birds,  401  ;  mam- 
mals, 436 

Nervous  tissue,  50,  55 

Nettling  cells,  173 

Neurone,    56 

Neuroptera,  296 

Newts,  379 

Nictitating  membrane,  352 ;  in  birds, 
401 

Nucleoplasm,  26 

Nucleolus,  20 

Nucleus,  20  ;  functions  of,  25 

Nutrition,  60;  protozoa,  147;  see 
digestion,  metabolism,  etc. 

Nutritive   pole,    32 

Obelia,  177 

Ocelli,  281 

Occulina  168 

Octopoda,  259 

Octopus,  257 

Odontophore,  244 

CEsophagus,  332 

Oil  glands,  323 

Olfactory  nerves,  349 

Oligochaeta,  231 

Ommatidium,  282 

Oniscus,  265 

Onychophora,  292 

Ophidia,  389 

Ophiuroidea,  214 

Optic  nerve,  349 

Orb-weavers,   304 

Organization,  18,  44,  60;  of  protozoa, 

147 

Organs,  60 
Origin  of  tissues,  56 
Original  home  of  animals,  129 
Ornithodelphia,  439 
Ornithorhynchus,  439 
Orthoceratites,  259 
Orthoptera,  294 
Oscines,  422 
Osculum,   159 
Osmerus,  366 
Osseous  tissue  (bone),  49 
Ostrea,  254 
Ostrich,  408 

Otocyst,  75  (see  also  auditory  organs) 
Otolith,   75 


INDEX. 


Otter,   443 

Ovary,  69 

Oviduct,  248,  342,  402 

Ovipositor,  268 

Oviparous,  342 

Ovum,  28 

Owls,  416,  417 

Ox,  442 

Oxidation,   15,  61 

Oysters,  237,  254 

Palcemonetes,  291 

Palaeozoology,  defined,   5 

Papilio,  301 

Papulae,   207 

Parapodia  219 

Parainecinm,   142,    146 

Parasitic  worms,  199 

Parasitism,   123,  190,  193,  199 

Parental  care,  108,  448 

Parthenogenesis,  108,  283 

Passeres.  422 

Pea-fowl,  413 

Pearls,   254 

Pearl  oyster,  254 

Pecten,  254,  255 

Pelagic  fauna,  129 

Pelecanus,   412 

Pelecypoda,  252 

Penguin,   411 

Pennaria,  177 

Perennibranchiata,   379 

Pericardial   sinus,   279 

Perichondrium,  49 

Periosteum,  49 

Peripatus,  292 

Peripheral   nervous    system,    74 ;    344, 

348   (see  nervous  system) 
Perissodactyla,  442 
Peritoneum,   314 
Periwinkles,  254 
Perodipus,  444 
Petrels,  412 
Pharyngognathi,   369 
Pharynx,   331 
Pheasant,  413 
Phcenicopterus,  411 
Pholas,  253 
Phyla,  key  to,   140 
Physalia,  179 
Physiology,  defined,  4 
Physostomi,  369 
Picariae,  419 
Pieris,  299 
Pigeon,  416 
Pigments,    10 
Placenta,  438 
Placentalia,  429,  440 


Planarians,   187 

Planorbis,  257 

Plant-lice,  296 

Planula,   176 

Plasma   (blood),  52,  336 

Plastron,  388 

Platyhelminthes,    185 

Plectognathi,  369 

Pleura,  335 

Pleuron,   262 

Plexus,  348 

Plover,  412 

Plitmatella,  185,  196 

Polar  bodies,   30,   31 

Polychaeta,  230 

Polygordius,  228 

Polymorphism,   180,    181 

Polyp,  170 

Polyzoa,  185,  195,  196 

Porcupine,  444,  445 

Porifera,   140,  156 

Porpoises,  441 

Potato-beetle,  302 

Prairie  dog,  444 

Premolar  (teeth),  433 

Primary  vesicles  (brain),  344 

Primates,  447 

Proboscidea,    443 

Proctodoeum,   60,   277,   330 

Proglottis  (segment),  190,  191 

Promorphology — defined,  3  ;  illus- 
trated,; 83 

Pronucleus — female,  30;  male,  31 

Prophase,  23 

Prostomium.  224 

Protective  resemblance,   119 

Proteids,   10 

Prothorax,  269 

Protoplasm,  9  ;  chemical  composition 
of,  10  ;  physical  structure,  10  ;  physi- 
ology, ii,  59;  relation  to  life,  8 

Protopodite,  263 

Protopterus,  370 

Proto-vertebrata,  308 

Protozoa,  140,  142 

Proventriculus,  277,  400 

Pseudopodia,    147 

Ptarmigan,  414 

Pteropus,  446 

Pulp-cavity   (see  teeth) 

Pupa,  285  (see  metamorphosis) 

Pyloric  caeca,  277,  314 

Quadrate  bone,  383,  432 

Quail,  413 

Quill  (see  feather) 

Rabbit,  444 


INDEX. 


479 


Rachis    (see   feather) 

Radial  symmetry,  85,  86 

Radiolaria,  153,  154 

Radula,  243 

Rail,  412 

Rana,  378 

Raptores,  417 

Rat,  444 

Ratitse,  408 

Rattle-snake,   383 

Rays,  368 

Razor-clam,  253 

Receptaculum   seminis,   227 

Rectrices  (see  feathers) 

Redia,  189,   190 

Reduction  divisions,  30,  31 

Reference  books,  466 

Regeneration,  89 

Remiges   (see  feathers) 

Renal  portal  circulation,  362 

Reproduction,  13;  of  cells,  22;  asex- 
ual, 68  ;  sexual,  68 

Reproduction,  organs  of — protozoa, 
149  ;  porifera,  161  ;  coelenterata,  175  ; 
unsegmented  worms,  184,  189,  191; 
echinoderms,  211;  annulata,  226; 
mollusks,  248,  249  ;  arthropods,  282  ; 
vertebrates,  341  ;  fishes,  366 ;  am- 
phibia, 376  ;  reptiles,  387 ;  birds, 
403  ;  mammals,  437 

Reproductive  epithelium,  46 

Reptilian  birds,  407 

Reptiles,  136,  380;  age  of,  381 

Resemblance,  general  and  special,  119 

Respiration,  62  ;  internal,  334  ;  in  pro- 
tozoa, 147;  coelenterata,  174;  an- 
nulata, 225  ;  echinoderms,  208 ; 
mollusks,  244 ;  arthropods,  278 ; 
vertebrates.  ^4  ;  fishes,  JUA.  ;  am- 
phibians,^£24  ;  reptiles,  384  ;  birds, 
401  ;  mammalsi436 

Respiratory  tree,   209 

Retina,  77,  78,  352,  353 

Rhinoceros,  442 

Rhamphorhynchiis ,  386 

Rhinoderma,  376 

Rhizopoda,  153 

Rhodostethia,  210 

Right-left  axis   (see  symmetry) 

Rodentia,  444 

Rotifers,  194,  195 

Round  worms,   193 
•  Rumen,  434 

Ruminant,   442 

Ruminant  stomach,  332,  434 

Sacral  vertebrae,  327 
Sagittal   section,   83 


Salamander,  379 

Salmo,  364   (salmon) 

Salvelimus,  365 

Sarcolemma,  53 

Saururae,  407 

Scales,    359 

Scallop,  254 

Scarabeids,  302 

Sceloporus,    389 

Schwann's  sheath,  55 

Sciurus,  445 

Scolopendra,  293 

Scorpionida,  303 

Scyphistoma,  177 

Scyphozoa,  177 

Sea-cow,  441 

Sea  cucumber,  214 

Sea  lilies,  213 

Sea  squirts,   308 

Sea  urchins,   203 

Seals,  443 

Secretion,   16,  61 

Sections,  defined,  83 

Segment,  275   (see  metamere,  somite) 

Segmentation  (see  cleavage,  meta- 
merism) 

Segmentation  cavity,  33,  34 

Segmented  worms  (see  annulata) 

Semi-circular    canals,    351 

Seminal  vesicles,  227 

Sensation,  70,  148  ;  see  also  sense  or- 
gans 

Sense  organs — protozoa,  148  ;  porifera, 
161  ;  coelenterata,—i75  ;  echinoder- 
mata,  211  ;  annulata,  226;  mollusca, 
247;  arthropoda,  281;  vertebrata, 
349  ;  reptiles,  385  ;  birds,  401  ;  mam- 
mals, 436 

Sensory  epithelium,  44,  45 

Serous  membrane,  42 

Sexual  cells,  28  (see  ovum,  sperm) 

Shark,  368 

Sheep,  442 

Sheepshead  (fish),  370 

Shell,  240,  241  ;  structure,  242 

Shipworm,    253 

Shrike,   425 

Shrimp,  289 

Shrews,  446 

Sight,   76    (see  sense  organs) 

Silk  worm,  300 

Siphon,  236,  251,  252 

Siphonophora,   177,   180 

Sipunculoidea,  233 

Siren,  377,  379 

Sirenia,  441 

Skates,   368 

Skin,    322    (see   integument) 


480 


INDEX. 


Skin  senses,   350 

Skeletal  system,  66 — in  protozoa,  154; 
porifera,  159,  161  ;  coelenterates, 
174;  echinoderms,  202,  205;  annu- 
lates,  223 ;  mollusks,  242  ;  arthro- 
pods. 276  ;  vertebrates,  324  ;  fishes, 
360  ;  amphibia,  373  ;  reptiles,  382  ; 
birds,  399 ;  mammals,  432 

Skull,   359 

Sloths,  441 

Slugs,  254 

Smell,  74,  350 

Smelt,  366 

Snails,  237,  254,  256 

Snakes,  389 

Snipe,  412 

Snowy  grouper,  370 

Social  instincts,  113,  287,  449 

Somite   (see  segment) 

Sow-bug,   265,  289 

Sparrows,  422 

Species,   102-103 

Special   senses,   74 

Spermatozoon,  29 

Sphincter,  •  333 

Spider,  266,  304,  305 

Spinal  cord,  348 

Spinal  nerves,  348 

Spinous  process,  325 

Spinus,  422 

Spiracle,  268 

Spirastomum,  148 

Sponges,  140,  156   (see  porifera) 

Spontaneous  generation,  153 

Sporocyst,  189,  190 

Sporozoa,  154 

Squamous  epithelium,  42 

Squid,   239,  257 

Starfish,  200,  205,  213 

S  tent  or,  148 

Sternum,  262,  327,  328 

Stigmata,  278 

Stimulus,  12 

Stolon,  176 

Stomodaeum,  61,  277,  330 

Stomach,   331,  332 

Stork,  412 

Streptoneura,  255 

Striate  muscle,  53 

Strobila,   184,   191 

Struggle   for  existence,  95 

Struthio,  407 

Sturgeon,  368 

Sunfish,  369 

Supplementary  studies — protozoa,  155  ; 
porifera,  164;  ccelenterata,  181  ; 
unsegmented  worms,  198,  199; 
echinoderms,  214 ;  annulata,  233 ; 


mollusca,  259;  arthropods,  271,. 306; 

fishes,  371  ;  reptiles,  392  ;  birds,  424  ; 

mammals,  449 

Supportive  tissue  (see  connective) 
Suture,  432 
Swallow,   422 

Swallow-tailed  butterfly,  301 
Sweat  glands,  323,  324 
Swifts,  422 
Swine,  442 
Symbiosis,  116 
Symmetry,  84,  368 
Sympathetic  system,  349 
Syrinx,  401 

Tacnia,  190,  191,  192,  199 
Tail,  358 

Tape-worm  (see  Tcenia) 
Tapir,  443 

Tarso-metatarsus,  400 
Tarsus,  269 
Teeth,  432 
Teleostei,   369 
Telophase,  23 
Tent  caterpillars,  300 
Teredo,  253 
Tergum,  262 
Termites,  296 
Testis,   69,   227 
Tetrabranchiata,   257 
Tetradecapoda,  289 
Thread-worms,    193 
Thrushes,  422,  423 
Thysanuran,  294 
Tibia,  269 
Ticks,  306 
Tiger,  443 

Tissues — definition,     40  ;     differentia- 
tion, 37  ;  classification,  41 
Toad,  378 
Tortoises,  388 
Tortoise-shell,   382 
Toucans,  419 
Touch,  69 
Trachea,  335 
Tracheae,  63,  270 
Transverse  section,   83 
Tree-toads,    379 
Trematodes,    187 
Trichina,  193 
Trivium,  205 
Trochanter,  269 
Trochosphere,  228,  229,  250 
Trophoblast,  437,  438 
Tube-feet,  207 
Tubifex,  232 
Tunicates,  309 
Turbellaria,   187,   188 


INDEX. 


481 


Turkey,  413 

Turtles,  388 

Tympanic  membrane,  268,  351 

Typhlosole,  224 

Ungulata,  442 

Unio,  234,  253 

Unsegmented  worms,  139,  183 

Upupa,   398 

Urea,  66 

Ureter,  341,  437 

Urethra,  341,  437 

Uric  acid,  66 

Urinary  bladder,   341,   437 

Urodela,  379 

Uterus,  192,  342,  437 

Vagus  (nerve),  347 

Variability,  94,  95 

Varieties,  102 

Vas  deferens,  227,  342 

Veliger,  250 

Veins,  64 

Venomous   snakes,   386 

Ventriculus    (stomach),   277 

Vermes,   183 

Vermiform  appendix    (caecum),  434 

Vertebrae,  327,  328,  384 

Vertebral    column,   325 ;    divisions   of, 

327 

Vertebrates,    137,   312 
Villi,  333 
Visceral  arches,  334 


Viviparous,  342 
Volvox,  151,  152 
Vorticella,  150,  154 
Vultures,   417 

Walking-stick  insect,  121,  124 

Walrus,  443 

Wasps,  303 

Water-boatman,   296 

Water-vascular  system,  207 

Weasel,  443 

Web    (of  spiders),   305 

Weevils,  302 

Whales,  441 

Whelks,  254 

White  ants,  296 

Whippoorwill,   422 

Wings — insects,  276  ;  birds,  395  ;  bats, 

446 

Wood-lice,  289 
Wood-pecKers,  419 
Wolves,  443 
Worms,  139,  216 
Wrens,   422 

Xyphosura,  303 

Yolk,  28,  29  ;  influence  on  cleavage,  32 

Zebra,   443 

Zoea,  284 

Zoology — scope,  2  ;  history  of.  6 


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